You don't state how big your dogs are or how much they are normally served, but, one cup of beans is a lot to substitute with their regular kibble - even for a large breed. This article notes that cooked beans are fine in moderation but it could simply be that the novelty wore off and they just don't like it.
Given the information you've provided, it is simply impossible to answer your question with anything more than speculation. Not knowing your routine with the dogs or their recent history, instead of answering your question definitively, I can only ask further questions: Did the dogs still have an appetite when they weren't interested in the beans - i.e. did the dogs eat kibble without a cup of beans on top of it? Do they seem healthy otherwise? Is their urine and stool a regular consistency? If they have lost their appetite or appear lethargic, what else do they have access to? In general, sudden changes of behavior indicate a problem but I am guessing the cause of this problem is simply that the don't want the beans. If the dogs are hungry, active and their stool and urine normal, then it is most likely they just don't like a cup of beans on top of their kibble.
That said, there are some simple guidelines to feeding your dog healthfully and frugally such that they both enjoy mealtime and get the nutrition they need in a form they can utilize. Put simply: unless you are a veterinarian nutritionist: do not feed your pet human food. Dogs don't want beans. They want meat. They need USDA and FDA approved kibble. The benefits of feeding them approved kibble are not only physiological (healthy immune systems), but behavioral (not encouraging begging for human food) as well. One financial benefit of doing so is that if and when your dog becomes ill, you can rule out expensive and time consuming tests attempting to discern the cause of the illness. For example - how much time have you spent just wondering why they stopped eating something most any veterinarian nutritionist would not recommend?
I don't mean to sound harsh, but what you like and what your dog needs are two very different things. I applaud that you are eating healthfully and want your pets to eat healthfully too. I also understand the need and desire to find affordable solutions to feeding your animals good quality food. That said, it is a bad habit to get into thinking that because what you eat is healthy for you, then what you eat is healthy for your dogs. Dogs and humans have very different metabolisms and physiological considerations for nutrition. Having said that, cooked pinto beans by themselves aren't poisonous, but, "not poisonous" is not the same as "a good regular source of your dog's overall diet". As I said earlier, the amount seems like too much. It's one thing to give them a treat, but meals and treats are also not the same thing. If you do want to give your dog a treat of cooked pinto beans, keep in mind that it is not the amount of treat you give them but it is that you are giving them a treat.
Dogs don't need the same kind of flavorings or variety of foods which humans enjoy. Dogs need meat. Find a dog food that they are okay with (i.e. not allergic to) which features meat as the first ingredient (and not grains or filler). I feed my 12yo Retriever "Pure Balance Grain Free" and it had a noticeable improvement on her health compared to the cheaper bulk food available at the same store. Note that Pure Balance includes peas in this mix. Peas, like pintos, are a legume. Not all legumes are created equal for dogs. Raw red beans for instance, are not such a great idea for dogs as they contain a toxin. If you aren't sure about which brand to go with, ask your vet for a recommendation. Dietary needs will change as they age.
The thing your dog will respond best to with their meal is a routine of feeding time and amount of food as well as a consistent kind of food. Dogs don't need a variety of food to keep them engaged in eating, or from getting bored with today's meals. This, along with routine exercise and play time, will maintain a healthy digestive and immune system. The result will be a healthy dog at an optimal weight. Your vet weighs your dog with each visit, right?
You can, of course, give your dogs treats and augment their diet with appropriate things like fish oil, minced parsley, 1TBSP chamomile tea, apples, carrots (raw carrots in moderation - too many can tear up their innards - cooked carrots are better), cooked chicken, cooked fish, cooked pork, cooked steak (all with no salt or seasoning and fat removed) or even a small amount of cooked, unseasoned pinto beans. Also, some foods like cooked sweet potato (no skins) or pumpkin puree or rice are good if your dog gets diarrhea. The thing is that the way you prepare food for you (e.g. with spices, salt, seasoning, etc...) is not how you want to prepare food for your dogs. All this said, I strongly discourage you from trying to replace the majority of your dog's diet with non-FDA and non-USDA approved dog kibble (unless you are a veterinarian nutritionist).
Also, this is a little off topic, but on the subject of feeding dogs human foods is also the issue of when and how you feed them human food. It is good training to keep your food and feeding time separate from your pets food and feeding time. That way, they are less likely to associate humans eating with them getting fed. This will make them less likely to beg and pester other people for food. Also, if you feed them snacks by hand, consider only feeding them with your left hand - most people are right handed and gesture with their dominant hand. If you feed them with only your left hand, they won't be looking to a moving right hand for food.
As for what to avoid feeding your dogs altogether, cooked unseasoned pinto beans aren't poisonous, but here's a list of foods that are arguably healthy or common, but are poisonous to dogs:
- Milk and Dairy (including yogurt)
- Garlic, Onions, Chives
- Raw/undercooked eggs, meat, bones
- Salt (& Salty Snack Foods)
- Anything with artificial sweeteners, added sugars, preservatives, etc.
Here's another similar list. Commit this information to memory. Get and keep your pets on a regular diet. Talk to your vet about what you are feeding your dogs. Also, make note of their stool - it should be a regular and firm consistency. It might be more rewarding to imagine that you are providing cost-effective food sources which your dog "loves" but instead of guessing about what your dogs "love" or "hate" sticking to a diet which is from USDA and FDA approved sources and with your veterinarians consent, you will instead have dogs that are "healthy" and "thriving" instead of "unhealthy" or "bored with food they used to love."