masurbation vs sex

So I am just curious...my bf (and rarely myself) can reject sex the night before but masturbate the next day. It doesn't interfere with sex( usually it's just to late before we get around to the deed ;) I am not saying he is addicted to porn or anything. Just honestly curious how it is different for guys. I am tired of feeling sad about it and I know if comes from a lack of understanding. Help!