Their experience with two cats in single household is that it ends with violence, including sexual, between cats when they grow up even when they were fine as kittens.
Not to discount your parents' experience, but in my own, I have never observed that--and I can almost guarantee my family (being rural-based and therefore needing multiple "working cats" as well as having a large number of "house cats") has gone through far more cats than they have. Intact males and females will mate, and to a human it may seem "violent," but that's just the nature of feline mating. Desexed cats who grow up together, absent any stimuli that might drive them apart, will continue to be just fine with each other as they age.
They are a little bit worried about problems with travelling (I don't want to tell them that but I hate travelling and even with one cat I planned to ask a friend to see him/her rather then introduce a cat to stress of international travel).
Avoid travel with a cat unless it's necessary. Cats are not dogs, and generally don't care for travel. They prefer to stick to their home territory, and take much longer to settle into a new situation than dogs.
They are also worried that, as I am single at this moment, I may have a trouble finding significant other.
This is the silliest excuse for not getting a pet that I've ever seen. Anyone who would be "turned off" by two cats will be similarly "turned off" by one cat, and in my opinion, isn't worth your time.
They don't buy the argument about kittens playing with each other when I'm away at work as they think that a single one will just sleep through the day.
Many rescues recommend taking two kittens instead of one, particularly bonded pairs of kittens, because kittens are higher energy than adult cats and do, in fact, play with each other during the day. A single kitten, on the other hand, will find whatever it can to play with, and this will potentially lead to household destruction. A bonded pair of kittens (whether siblings or otherwise) will do a great job of keeping each other occupied, and while they may still find mischief to get into (my pair learned to work together to control my thermostat!), it's considerably less than what a single kitten gets into (I had to keep my singleton in a borrowed dog crate to keep her from destroying my bedroom).