That's what I was thinking. 37°C inside vs. up to 32°C (90°F) outside and you still have enough temp gradient to produce the power. All you need to do is install this heatsink on your chest :)

And to satisfy precedent, pipe any leftover power after running the pacemaker to a ring of white LEDs.

Testicles versus main body is the classical example. Sperm production and conservation just works better around 32°C.

On a more practical note, I wonder how feasible a thermoelectric powered earpiece would be...

How does that works? For a period of time before you try to get someone pregnant you should try to keep your testicles at that temperature to get the best quality possible?

Yes. Boxers. Loose pants. No hot baths or saunas. Good luck!

There are likely papers on the exact effect over to be found on google scholar.