TV Tropes explains the (Infinite?) Mermaid Problem:
Alternatively, a sufficiently bawdy work—which one is likely to be if it brings up the Mermaid Problem in the first place—may simply point out that lower half aside, well... She's still got a mouth, ain't she?
Oh, duh, oral. I was assuming "have sex" didn't include oral.
Anyway, my answer is still the same.
Thanks for the link on the mermaid problem (and I managed not to open any other TV Tropes pages somehow!)
This is our monthly thread for collecting arbitrarily contrived scenarios in which somebody gets tortured for 3^^^^^3 years, or an infinite number of people experience an infinite amount of sorrow, or a baby gets eaten by a shark, etc. and which might be handy to link to in one of our discussions. As everyone knows, this is the most rational and non-obnoxious way to think about incentives and disincentives.