This whole idea that "I have to know if I'm sexually compatible with him before I say yes" is utter nonsense. Dude, the parts work, and you're compatible with anyone physically. It's the mental and emotional compatability that matter, and you can't force that no matter how beered up you get.
So yeah, getting frisky before marriage is bad. You could end up with an unplanned pregnancy or something. Nothing worse than having a shotgun weddding 5 months pregnant, or having him call it off once you get a positive test.
Yes, best to wait to get frisky until all the legal documents are signed and you can procreate without worrying about redneck parents and their shotguns
I'm talking to you like a non-Muslim from the non-Muslim perspective, cuz I can't imagine a Muslim asking such a strange question.