And how can Mulder's face be both serene and full of regret?

I stumbled over this, too. It seems a contradiction in terms, and beyond that I can't picture Mulder regretfully having sex with Scully. To me, his reaction to finally coming to this point would be one of, well, characteristic exuberance. :-)

That said, I liked the fact that she took a more original (and thematically tied-in) direction with this story rather than echoing the approach seen in so many post-*all things fic.