Yeah, that's what I thought when I first saw it, and I think it's right now. Somebody elsewhere on the web said they at first thought it was cheating, which got me thinking about the fact that Janice cheats on Chandler after they get back together not too long before that. However, that doesn't quite fit, because Janice cheats on him, not the other way round...
Anyway, after overthinking this way too much, I'd say the answer is erectile dysfunction when you at first get back together with an ex.