I hope that you're maybe 16 or something, you've been "dating" a girl for a bit but no sex yet, and you're asking about how to move things to the next level because you're both virgins?
Anyway, the general answer to your question is NEVER.
You can talk about sexual topics to show your confident and experienced and are comfortable talking about these things, i.e. as part of a generally fun, flirty conversation, and you can use sexual innuendo as part of your banter to again show confidence, playfulness and sexuality in the right way.
HOWEVER, you must NEVER bring up the subject of sex in a serious way, e.g. "do you want to have sex with me" or "I think we should sit down and have a serious conversation about taking our relationship to the next level" UGH.
Women HATE responsibility. They want a man with the confidence to take control and lead the situation forward, whilst at the same time shouldering all of the responsibility. She never wants to feel like she made the decision to have sex with you, or initiated the sex because then, according to society, she's a slut. She wants you to lead things forward in a way that's completely comfortable for her so that she can say "it just happened" like it wasn't her fault or her decision, but she just went along on this ride.
The way you do this without saying a WORD, is to kino escalate using push-pull, past friendly touching to sexual touching, always being the first to break contact and leave her wanting you to re-engage and escalate. If you leave her wanting more each time, then she'll get hotter and hotter for you and practically beg you to stick it in her
CHAT
