I'd go for it for as long as it takes to get them out of the main room and into a private space, where he cleverly explains the deception and then talks her into working with him to achieve some plausible and noble goal. In the process of regaining her trust, they then later can have the brilliant sex, etc. Basically, I agree with Captcha.
I think I've got all those elements - having great sex, admitting the truth, working together to achieve something and gaining her trust (can't say regaining because he never had it in the first place) - just not in the same order that you mentioned.
It's pretty clear that the scene as I envisioned it will turn some readers off, but that's just something I'll have to deal with. At least the scene will be at the start of the first chapter, so hopefully readers will know right away whether or not they want to go on.