What is she getting out from it? Remember that for them sex is more a hassle than fun. Some girls really love sex, but most are just using it to get something in life. She is giving you sex so she can get something back. What do you my friend give in return for sex?
You bring a girl home, everything is going smooth, you open a bottle of wine, play some music, you are enjoying the conversation with her, she loves it, you start kissing and then in one moment she says the killing sentence:
“Just for you to know, we not going to have sex tonight”