You are approaching it in the wrong way!
First you can't never give room for a girl to tell you this kind of stuff like... "We are never going to have sex"!!!
If they tell it meaning they are full of them self and none other girl in the group will have sex with you so cut her off.
I am a girl but this is just a bitch acting and speaking in my opinion! We have to respect people and if a boy wants to have sex like all boys we can be respectful and sweet about it. If she was mean with you it's because she feels better than you, cut it off right the way!! responding: since when I am interested to have sex with you? get a grip!
You don't ask for sex! sex happens in a good moment together having fun, in a lonely moment, in a horny moment so it has to happen and you have to go for it! sometimes break the girl's ice and act.
My brother used to say! I will ask just to put my head because my rest it's just a neck! do you understand? silly but works.