Curvy and sexy girlfriend - she is insecure

My friends are making fun of me because I started dating my current girlfriend who is a bit curvy.
I am a fit guy, I am going to the gym 5 times a week and, without any modesty, I can say that I can date almost any type of girls.
But, I like her the way she is and I am not ashamed of going with her in public.
I am more than fine with the way she looks like, I wouldn't change her. I can't see why so many people are telling me to dump her.
She noticed that my friends are making fun of her and she became very insecure about the way I feel about her. I told her that I love her body and I wouldn't be with someone else.
Also, when we are having sex, she is afraid of being on top of me, because I might not like it. But I am finding her very sexy!
Why do men think that the slimmer the girl is, the hotter she is? For me, it's really not like this.

PLUS - a few nights ago we were talking about the swinging lifestyle (we both were in the LS before meeting each other) and I told her how much I want to do it with her. She went like “ok, why not?”, but I could feel the insecurity in her voice, so I asked her what's wrong. I told her that I won't do anything without her, I did not want to put any pressure on her.
Later that night, she admitted that she is afraid of getting into the LS with me because I am sexier than her and she is afraid that no other man will find her attractive.
I really don't know what should I tell her to make her feel better.
I have tried in all the possible ways, but she doesn't believe me when I say that she is beautiful and sexy.
All, you can do is to keep trying, as long as you feel comfortable with that. There is no magic solution.