Huh????
You're already having sex!!!
(Is this Bill Clinton in disquise??? Really...)
We all have a line that once crossed, we know we're "in like Flynn". For some guys, they know that if she lets the guy touch her, he knows that he can bang her that night. Some know that if the kiss goes well, she's getting banged that night. And then, (99.72% of) guys know that if she's willing to suck your c0ck, you will be banging her w/i 30 minutes (you have to decide when to pull her off your johnson there...)
I usually know based on the kiss. The touch thing I haven't refined enough to tell, except in a few cases....but if I'm getting blown, it's so much a done deal that I don't even wonder how lucky I'll get that night.
If you can get blown but not laid, then I have to wonder about your bedroom technique.
I hope you don't ASK to mount her??? (A girl at work told us "if a guy has to ask to kiss me, then forget it, he's already wrecked the moment, I am so outa there..." Multiply that by 10 for getting laid. You merely escalate the moment---go from oral to whatever position you want as if it was perfectly normal, without apology or asking permission and lead her into it. If she pulls back, resume what you were doing or something new that drives her wild until she's begging you for it.
As for ruining a perfect moment--this is putting too much on yourself, don't even think that way. This is a self-induced mindfvkc. It's as if you're thinking there is only one way to pull this off and enjoy an intimate evening with her. There's a hundred ways to get her stoked and enjoy it.
I'd say don't bring it up, just do it. Others would say that you need to discuss it. I've never seen that work for me---what they refer to, I think, is being prepared (birth control, but I don't need to say that, do I?) and "bringing it up" is no big deal if you're prepared. I don't bother with "bringing it up", it happens or it doesn't and it's usually clear why.
Your girl is probably wondering why you haven't been aggressive enough yet.