Ah, poor River. You could write a companion piece, in which she contemplates the tragic fact that, while for him it's the first, for her it's the LAST time. And I think his sweetness and shyness would fill her with a terrible and painful tenderness.

It's still really odd for me to think of the doctor having sex (though... yeah! about time, old man. lol) I grew up with asexual doctors, after all. (Though Fourth Doctor? Yum yum!) But this I really, really, really liked.