the main appeal is good old-fashioned denial. most people, especially men, are so insecure about their own sexuality that they panic at any potential "challenge" to it.
this presents a problem for their fragile egos, because as human beings we are programmed to respond most strongly to visual stimuli, and the more intense the stimulus the better the response - i.e. to the average healthy member of the appropriate gender, a guy and a girl going at it hot and nasty is generally more arousing than either a guy or a girl on their own. this isn't on the level of preference or even specific sexuality; it's just that our brains are structured to process information in that way. when you happen across sexual media of this nature, your brain sits up and takes notice even if you've no physical interest yourself (i.e. straight sex if you're entirely gay).
however, the modern straight male cannot ever admit, even to himself, that he might derive some titillation or arousal from anything remotely associated with another man's body. to do so would open up the issue of self-exploration, something most human minds are simply too shallow and flimsy to support. even in cases where they're not, comprehensive indoctrination / peer pressure will ensure that the sight of another male's genitals - or even the potential risk of such - will cause an actual fear-aversion response (not on the level of a panic response by any means, but enough to cause an appreciable level of discomfort).
pathetic isn't it?