Sex positivity means many things, but it does not mean that all sex is positive or enjoyable.

Sex can be agreeable, but it’s not a necessity of life.  It’s not something you have to give someone else.  If you aren’t interested in it, you aren’t abnormal; you aren’t strange, you just aren’t interested in it right now.  You have the right to continue to feel that way or toe change your mind.  It’s your body.  If you’re scared, if it hurts in a way you don’t like, say No.  Keep saying no until the other person stops.  If s/he won’t stop, start screaming no.  You do not have to keep having sex if you aren’t enjoying yourself.  You don’t have to have sex in exchange for dinner and a movie, I don’t care how good the restaurant was.