If their SOs aren't doing what they want, either they haven't talked to them, or they have a shitty SO, although it does not justify cheating. If you enter a relationship, you should respect it from start to finish. If you want to cheat, break up before hand, otherwise you are being a terrible person.
Sure she can do whatever. But a relationship is a contract, and you sign it. You can no longer do whatever you want. If you think otherwise, you'll end up a sad old person
