Nice thread...
AFAIR its simple caveman psychology (if you bear in mind that we are still wired like them and that sex basically is meant to propagate your genes)
a) well-defined body parts equal health, and in the competitive male mind bigger is better.
b)if someone just fucked your woman, best strategy is to fuck her again yourself and so "flush her out" and replace the other guys sperm.
=> if the other one looks better, healthier and whatever, you have to try harder
so: you cum harder, too, than with a sicklish-looking, limp-dicked guy with a beer-belly...
Even if "your Woman" is only Miss Hand and her 5 daughters, same things apply...
And the same thing happens with things like MMF threesomes, gangbang or other stuff: the more concurrence, the more you shoot...
Sexual psychology is fun... BTW, did you know that a woman is more prone to cheat when she is fertile, and that she will get pregnant a lot easier by a strange man than by her partner?