People are making fun of me for my girlfriend? What should i do?

So im 14 and i've known this girl a few months now and we starting going out last week. I really like her and she's just amazing, but my friends for some reason are like, hey are you going out with her? and i say yea and they're like, EWWW MAN. SHE NASTY. its not like she's ugly at all or anything so i don't get why they're saying this but it's really really really bothering me. especially cause i don't do anything about so i feel like i must be a real to not stick up for her. =( what should i do?