Whenever you're dealing with understandable cases like this (which is different from e.g. cats that can't stand each other), you need to always be aware of what the baseline is.
If you have a dispute with someone else, and cannot amicably resolve it, you go to court. As there is no amicable compromise possible, the court then resorts to the applicable legislature, which is essentially the legal baseline of how such a conflict should be resolved.
Cats have no knowledge of the legal system, but your male is essentially "suing" the female for taking away affection that he feels is rightfully his. Regardless of whether he's correct or not, he thinks that he is correct.
This is what I use as a baseline for my three cats:
All cats are equal, and are entitled to equal shares of everything (treats, toys, affection, ...)
You can define your own baseline if you want. Just make sure that you are absolutely convinced about the fairness of your baseline, because you will enforce this baseline unilaterally.
A bit more off-topic, but relevant to understand why I chose this baseline:
This is largely based on the categorical imperative (as defined by Immanuel Kant), a philosophical idea that I very much believe in. Simplified:
The freedom of an individual should only be limited when it (unfairly) infringes on the freedom of another individual.
In other words, you are free to do as you please, but you cannot detract from someone else's freedom, within reason.
The baseline defines what each cat is entitled to, but that doesn't mean that everything must adhere to the baseline.
If we sit down on the sofa, he usually jumps up to lie down next to us. The female is a more stereotypical cat: she enjoys some physical attention every once in a while, but is just as happy lying in a chair by herself.
The male clearly wants more attention than the female. And that's not inherently a problem. It only becomes a problem when the female loses out on attention that she wanted.
If the female is simply not a social cat, and does not ask for attention, then she does not need the attention. If, at the same time, the male is asking for more attention than he is entitled to (according to the baseline), then you can divert attention towards him.
Note: There is an exception here: if the female wants attention but is afraid to ask for it, e.g. if the male attacks her for doing so or she is pathologically afraid of engaging humans (I had a cat like that). In such a case, you need to rely more on the baseline, in order to show to the female that she deserves attention too.
My girlfriend and I try to give the cats equal attention, but I always ended up giving the male more because, well, he was usually at arm's length. My girlfriend has a bit less patience for him (he's really needy = annoying), though, and so gave more attention to the female than I did.
Both of your approaches are valid for different reasons.
- You gave more attention to the male because he was easier to give attention to. As long as you did not allow the male to undercut the female, that is fine. It teaches the female that she's more likely to get some additional love by being around you. Otherwise, you are setting a dangerous precedent that you are required to seek out your cat in order to give her the attention she wants, and that's not right. If she wants something, she should come to you.
- Your girlfriend, however, finds the male's continual presence annoying. Therefore, she responded fairly by not giving him more love just because he's closeby. She effectively teaches him the opposite: he is not going to get everything he asks for, just because he's close. This too is a valuable lesson, and it subtly teaches the cat to adjust its behavior based on what you prefer.
The only real issue here is when you cross wires. E.g. your girlfriend tells him off of being close to you (not her), or you start ignoring him more only when your girlfriend is around.
This sends mixed signals to your cats, they can't figure out whether you like their proximity or not because you seemingly flip-flop.
Consistency creates patterns. Patterns can be recognized. Recognizing a pattern leads to understanding them. Understanding patterns leads to the ability to steer your own behavior in order to achieve the outcome you want.
A simple example:
- If your cat scratches the wallpaper, you spray him with water. You do this consistently and preferably without fail.
- Eventually, the cat will understand cause and effect. Scratch the wallpaper 🡪 get sprayed.
- The cat does not want to get sprayed, but he does not know how to avoid it. Depending on how clever your cat is, it may take him a while before he realizes that he can avoid being sprayed.
- Eventually, the cat learns to steer its behavior based on understanding cause and (inevitable) effect. I do not want to get sprayed 🡪 I should not scratch that wallpaper then.
Every animal (and human) is capable of this learning pattern. It's very natural to do so, because it's a universal law of life. If I don't want the effect, I should avoid the cause.
The tricky part lies in understanding that a certain effect is connected to a certain cause. This is where you must help your cat. By consistently (and fairly) punishing him, you make the connection between cause and effect easier to spot.
The female has now become significantly more affectionate, and the male isn't too happy about that.
This is where we get to the "court case". The male feels that the female's increased need for attention is unfairly infringing on the attention that he's used to getting.
You shouldn't punish him for jealousy; because he's not being malicious.
What you should do, is remind him of the baseline. He has gotten too big for his boots, and needs to be reminded that a lot of the attention he's used to getting was an extra, he was not entitled to it.
Just like how the court relies on the law during a trial, you should rely on the baseline during the male's disagreement with the amount of attention he's getting.
Since there is no amicable compromise anymore, you must enforce and police fairness. Try to ensure that affection is given in equal measure. It may seem childish to do "one pet for him, one pet for her, ...", but keep in mind that cats are less intellectually developed than the average child, and this childish solution is understandable to them.
We had a similar issue with our first two cats. They were sisters, but one would always steal the treats and food of the other. So I enforced fairness. One treat for Cleo. One treat for Misha. Cleo would not get a treat until Misha ate hers. I physically blocked Cleo from stealing it. If she did manage to steal it, she forfeited her next treat. Cleo had two consecutive treats, so now Misha shall now have two consecutive treats. No discussion, I'm the one who makes the rules.
It took a few weeks, but Cleo got the message. She never steals treats from Misha anymore. Note that if Misha walks away (she has done so on occasion), I allow Cleo to take the treat that Misha clearly refused. But only if Misha refused it, and if Cleo had not already tried to steal it.
We recently got a third cat, who is even worse than Cleo at always wanting to be first. I've applied the same rules. He has also understood the message by now, although he is still a kitten and sometimes is too excited to remember the rules (it's not malevolent, he's just eager).
He will get pushed aside if he tries to steal a treat. I consistently prevent him from breaking my rules, and he has over time become less adamant about trying to get around me or my rules.
For the last couple of days, my girlfriend has reported that if she's lying in bed with the male and the female comes in looking for some affection, he bites her (the female).
My girlfriend states that she has not noticed such behavior in other situations; only when the female is "interrupting his affection-time".
His actions have scared off the female. This makes it impossible for you to enforce fairness, since the female no longer wants affection as she has been scared off.
If you cannot provide fairness by giving her equal amounts of affection, then the only remaining option is to provide fairness by giving him less affection.
He gets jealous about her wanting attention? He has now earned himself a strong negative feedback (stating a clear "No" while making eye contact with him) and an "affection time-out" (no affection for the next X minutes).
He chases the female off the bed? Then he's not welcome on the bed either. If you can't share it, then you don't get any.
Being jealous for attention is one thing. Attacking the other cat (even if only symbolically) is a crime, and you must teach him that crime does not pay. Whenever your cat commits a (social) crime, you must ensure that he gains nothing from his crime.
Note that you can't be too hard about this. When the male gets kicked off the bed, he's not sure what happened (especially the first time). He may wrongly think that you don't like him anymore. Prove to him that that's not true. Make a point of giving him attention at the same time as the female.
Provide tangible experiences for the male, both about the positive (both cats get attention) and the negative (you don't get to claim or steal attention).
Some examples from my cats:
- Cleo wanted Misha's cushion. Misha was sleeping on it. Cleo attacked Misha, drove her off, and got comfortable on the stolen pillow. I picked up Misha, brought her back, shoved Cleo aside (gently but firmly) and put Misha back on the pillow.
- Cleo repeatedly stole Misha's spot. Misha didn't want the spot when Cleo chased her off, so my previous fix no longer worked. So instead, I removed Cleo from the pillow and dunked water over the pillow, rendering it unusable as a sleeping spot.
- Cleo eats faster than Misha. She also eats more. When finished with her bowl, she would butt in to Misha's food. I reprimanded her twice. The third time, I took away the food for 10 seconds, put it down in front of Misha, and blocked Cleo. (Note: Misha never finishes her bowl. If she walks away, I allow Cleo to eat the leftover. But again, only after Misha voluntarily stops eating).
The core idea is very simple: You gain nothing from being unkind. If anything, you will lose something. Even if that means that both cats lose out (e.g. when I dunked water on the pillow), I will not allow their crime to pay at pretty much all (reasonable) costs.