Why is it that people attack our love interests to hurt us?

Recently because I have introduced my girlfriend of 5 months to my family all I hear from my cousins is shes ugly, shes has no , she has no behind. I sometimes wish i could be far far away from my family. The words of my cousins get to me because I love her and she is absolutely gorgeous to my eyes. I am okay with them having their opinions but keeping to themselves would be much appreciated. But no the onslaught of mental abuse continues. Even in the past with past girlfriends they had slandered and put me down. How can my own family not enjoy the fact that I am happy. Last summer was horrible for me because of many a things. I was just so distraught that nobody loved me. But now she does and all my cousins want me to feel is misery. It makes sense that they make fun of her though. She is a very self-respecting individual who does whatever she wants just like me. We do not follow the foolish trends of modern teens. So what she can stick up for herself. Whatever. This is a new age and women deserve the same freedoms as I. I just don't understand how my cousins can do this to me while they find girls who are in my idea ... how do you say somewhat promiscuous. Please guys give me whatever answers you can. I just wanted to vent and I know that it won't change my cousins feelings about my girlfriend.