Sexual chemistry is something that happens immediately when you meet someone in the real world for the first time. Not all sexual chemistry is a wildfire of "let's get it on." But most of the sexual tension and sexual awareness comes from recognizing your own desire, your own needs, and how you imagine this person fitting in and even pushing the envelope a bit.
Our children have probably learned more about relationships (good and bad) from television and porn. It's part of why all of us are so confused about what makes for a healthy relationship.