Unfortunately this game is very complex and not everyone has the same feelings about what is cheating/cuckold etc.
I can tell you that you are not the only guy in town, that many girls are in relationships with other guys, and that other guys will try to have sex with girls you like.
There are different types of relationships (Girlfriend, just sex etc...) and not all girls are available for every kind of relationship. some will only be interested in sex others will only be interested in a serious relationship. Others may start out as "just sex" but then the relationship may become "serious", some want just sex but will still be loyal to you (if you don't neglect them) even though they don't want a serious long-term relationship. If you have a serious relationship with a girl and you don't neglect her she will not cheat on you... There is a part at the beginning of the game, during a party, where MC gets a little humiliated. That part will however be changed in the next release giving more options to MC.