Theres 0 excuse for cheating, guy, girl, gay, straight idc. No excuse. If youre in a relationship that you feel neglected or want to date someone else or sleep with someone else WHATEVER the case you need to man up and end your relationship. But some people dont give a d.a.m.n. and dont care how many people they hurt.
There are multiple different reasons why people cheat. To be spiteful, because they feel neglected, etc.