1. People are generally taught in modern societies that monogamy is the right thing to do. Polygamy is typically taboo.
2. Some men are taught to believe that if they wait for the right one and stick with ONLY her, that's going to be the way to win her over. And there is a fear that goes along with that belief. Because the guy believes that if he went with a bunch of girls, that will ruin it for when he finally meets the right one because she will reject him because he had other girls.
3. Some men are afraid of diseases.
4. Some men are lazy and don't even want to get off the sofa.
5. Some men can't or think they can't, and that's what keeps them from doing so.
6. Sex is shamed upon in some societies.
7. Etc., etc..
If you read over the entire list of reasons that I just listed, none of them have anything to do with the man's actual desires. They ALL have to do with the guy "thinking" he can't or shouldn't for one reason or another. It's all mind control.
And, if you did get with him and gave him 100%, and if he knew he could have other girls, he would get bored of you and possibly eventually move on or start having interest in other women.
LOL!!! Apologies if that was too graphic or insulting. No offense intended. I'm just trying to be realistic here.
Most guys will if they can. The real exception might be a happy family life, because that's different and children are important.
A lot of men are monogamous because they are afraid of losing something good. Sometimes it's mediocre at best. That's why some men marry ugly mean women who cheat on them and who already have kids from other men, and then they take care of the kids like their very own.