Another one ...
You guys must hate me. I can't help it though, I keep remembering things I wanna reread and can't find ... anyway, this one was J/B slash, and in it Jim was totally bottom. In the story he told Blair that he used to fake it with Carolyn by gettin her so worked up she'd come immediately after he thrust in, so she didn't notice that he never could keep his hard-on. Until she did, and made him see a doctor, and the doctor told him he should seriously consider changing orientations - but I don't think he had, in the story, not until Blair.