If you were my daughter or sister.., I know this is a little hard to swallow.., but I'd never let you date someone like that..
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..
If my daughter or sister didn't realize or mind that at all.., then no less could be said about her..
I can only say.., try and control yourself.., try not to get too carried away from the excitement and trill of dating someone older and feeling as if you are somehow more mature for being interested in older guys or by virtue of association.. You're not going to be the first or the last girl who has felt this way..
I would try and seriously re-assess the situation before you run to your family and tell them.. Try and think about it without the whole "but I love him" popping up in your mind.. I didn't say he's not a great guy.., but I would be skeptical of someone 21 who is fine with dating someone 17..
If you can't stop the Wind, then you can't stop the Storm.