OK, another of those didn't-sleep-a-wink-and-my-mind's-gone-to-a-weird-place kind of questions. Final Season spoiler, folks....
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In, I believe, TOW Joey Speaks French Monica tells Chandler that "Shovelin' Joe" couldn't be the father of their adoptive child, because "you can't get pregnant doing that," Chandler asks, "The thing we hardly ever do, or the thing we NEVER do?" I feel fairly confident that I know what the two things are, and even which is "hardly ever" and "never."
In the next new episode (Don't know the official title), Phoebe's hubby "Crap Bag" freaks out when Phoebe starts talking about the thing he won't do in bed. Then when she whispers it to Joey, even he freaks out.
...Now, I've been wracking my not inconsiderably perverted imagination, and I cannot think of a single thing that a guy would say no to if a woman asked, at least if she asked nicely. Maybe it's just me. Anybody care to share?