Well before I can answer that I have to ask you what you mean by okay. If you mean morally okay that's one thing but if you mean okay to you that's another. Guys know that having a side chick is going to upset you. They're not delusional to the idea that a side chick is not okay... TO YOU. I'm not in any way saying having a side chick is okay. I'm just pointing out the difference between what's okay to them and what they know is okay with you. So the easiest answer to that part is they know it's not okay with you they just don't really care. Now if you're asking why it feels okay to them then that's a little more complicated. First of all is morality. Morality, although we like to believe it is, really isn't set in stone. Everyone has a different set of morals. But beyond that actually listening to an abiding by your morals is still a choice. People don't always choose being "good". In fact a crap ton of people go about life with the belief that nothing is wrong, or at least it doesn't matter, unless you get caught and even then if the consequences really aren't that bad they'll do it anyways. So a guy with a side chick already has two women interested in him. If say one of the girls breaks up with him he just moves on to the other girl. If he really doesn't care about that particular woman and just wants A woman then the consequences for having a side chick really isn't that bad. And if he really doesn't care about that particular woman and just wants A woman he doesn't have much of a reason to care about whether or not she cares about him having a side chick. You may not like that answer but I know enough players to know that's the truth.
I don't believe that all men think that's okay. That's called cheating which many people disapprove of. For the ones who do think it's okay, it's because they're selfish and care more about themselves than their partner. They want to have their cake and eat it too, which obviously does not work. Also keep in mind that this isn't just men. There are plenty of women who think it's okay to cheat on their boyfriends/husbands too. It goes both ways.
There's men and there's boys men don't have side chicks because they want to date a good woman. Since a good woman wouldn't tolerate that and since a good woman is hard to find a man would never risk it. Now boys on the hand would mess around with side chicks. They just haven't matured yet that's why they are still boy with chicks not a man with a woman.
You make this sound like all guys think a side chick is ok. We do not all think that. Most of guys think a side chick is not ok. I know I think it is not ok and all my friends think it is not ok to. Maybe you are meeting guys at the wrong places.
Not all guys do. Some guys cheat, some girls cheat. People cheat because they aren't fully satisfied with their relationships, not saying that justifies their deceit and hurtful actions, but that's why they do it.