I think its cos they dont really love the girl they're with or they dont fear God. Either way its still wrong. if deep down, they want to see more than one girl then its stupid of them to agree to a committed relationship. Be a man and tell her you dont wanna be tied down, at least its being honest, and you wouldn't have the chance to cheat on her.
With that said, however there are guys who can be trusted not chase afta some ova girl while ure not around.