I'm not sure I've got a copy of Surprised by Joy around, although I have read it. What is his analysis?

It may not be the end of the matter for me, for all I know. robhu's recent reconversion reminds us that none of us can be lax about these things, lest we backslide into theism.

I think the theory of moral contagion isn't about bad company, it's about uncleanness as a thing that can get passed on without an obvious mechanism (so I don't want to wear a shirt worn by a murderer, or whatever). With bad company, there's an obvious mechanism.

There isn't really an argument on that page, just a parallel between two things that modify their hosts' sexual behaviour for their own benefit.