I find darker skin to be exotic, which is one thing, but I am also uncut with light skin, so another perspective...
I find a lot of the people I know are repulsed by the thought of an uncut cock if they have not experienced it; I think this is because humans are naturally uncomfortable with things they don't understand or envy. Others I know love the thought of an uncut cock; I think this is because it's less common (here), and that's alluring in most circumstances; humans value things of scarcity (as long as they aren't afraid of them). For example:
Rare gems and minerals
Antiques
Exotic Cars
Collectors Items, even if not costly (the real ones)
Outstanding physical fitness
Unusually perfect symmetrics.
Uncut cocks ;-)
I also wouldn't say this is just gay men, but I guess men are often more visually stimulated, so are more likely to objectify a cock.