I'm not saying it's true all the time.., but a guy who is 21.., and is considering dating someone who is 17.., is either still naive or desperate.. Those are both not qualities in a guy I'd like my little girl or sister to be dealing with..
If he's 21.., then why isn't he at the very least with someone 18? If he doesn't feel guilt or shame or a little bit like a loser or pedophile.., if he expects to have something serious with someone that young.., then he's still naive.. If he's aware and feels all of those things and still doesn't mind going ahead.., then he's desperate..