but it necessitates something I'm not sure I understand on Simon's part--sexual desire overpowering rational thought and the awareness of all the reasons not to have sex with his sister. Intellectually, I do know people make stupid choices about sex, but it's not something I can really grok. Maybe I'm a closet prude...

Heh.

I don't think it's prudery, but I do think that in Simon and River's particular case, there are lots of circumstances that might make Simon give in even when he knows it's wrong, and not all of them have to do with desire.

If it's something River clearly wants, he may feel so guilty about her condition (and his inability to get to her before it was done to her, his at-the-time inability to heal her, etc.) that he will do *whatever* she wants. After all, he's only really hurting himself, right?

The mind is amazingly flexible when it comes to rationalizing away things we know are wrong, especially when someone we love is in pain or need.

I definitely think it's one possible reading of the story, and of his character. He may not even realize he's doing it, either, until afterward, which will make him feel even more guilty.

Lovely circle of angst, there. *g*