I'd bet 'snakes' gravitate towards your crotch. sorry, had to be said.
but seriously, it's not really one of the warmest areas, actually the opposite. testes need to remain at a pretty specific temperature slightly below core body temp or sperm production is affected -> infertility.
having said that, i completely agree that i can't see why anybody's penis is getting cold. only happened to me this one time, when there was an ice carving of a naked chick, bent over...