Most Helpful Girl
Because they aren't very clever. Some guys are also used to girls just saying she has a boyfriend to make him leave her alone. Then there are guys who think they can encourage the girl to cheat.
Most Helpful Guy
Many girls aren't loyal anyway. They tend to flirt when they are with another guy to shop around for a new man... It's not smart to commit to girls like that, but if you're looking for sex girls like that are fair game. Just don't let your feelings get in the way, because they'll move on to the next guy behind your back just as easily.