She likes to believe that, even though he's cheating on his current girlfriend for her, that he'd never screw her over.
The other girl is commonly written off as slut or whore by those who misunderstand her, when really, she just has a drafty vagina needs to be kept filled.
You're.... THE OTHER GIRL.
You're okay with letting yourself get used by a guy with a girl friend!
Don't worry, he loves you, and he'll eventually be with you, when his girlfriend dumps him.
Until then, he'll have you on a string, but that's alright, because you're okay with being used for sex because you like feeling wanted.