I am sad about my girlfriend going to her best friend's lingerie party.
Why are you sad about her going to the party? Were you not invited?
She says men are not allowed and she doesn't want me to see half naked girls.
It's understandable to feel that way and wanting to see half naked girls. But it's a female only party and it would be weird for your girl to bring you with her.
I'm never going to have sex again.
Do not take it personal, just know that sometimes girls need to have girls only parties. Just like sometimes guys like to have their time with just the guys.