So I don't understand not wanting to have done it if you wanted to do it in the first place as long as it was, in fact, fun.

If sleeping with someone otherwise attractive would be BAD BAD VERY BAD in terms of the repercussions, I would want to do it but not want to have done it.

I had a chance to make that mistake once and, to my everlasting relief, did not make it. Oddly enough, common sense overruled lust in that case, and I'm arguing for lust's superiority over intellect in this post.