I think there's a lot of reasons why gays have a reputation for promiscuity and infidelity... and they're all right.
One factor is that gay relationships are already outside the convention and mainstream, thusly making more attractive the alternative routes of pursuing relations with others, up to and including what we call "cheating" or "promiscuity".
Our moral economy is not similar to the conventional heteronormative (to don a monocle and use a term). This also makes open relationships seemingly more common (or visible at least) among gay couples.
Secondly, guys are pigs and we love sex, and sex is the easiest thing for a gay guy to find. It's the relationships that last more than six weeks that are impossible.
The reason for this is cultural, in my estimation, and many of us are hugely influenced by cultural (or peer, or family, or whatever) pressure. Sex is easy for us to find, so our culture takes on that and those who enter are treated, for lack of better terms, as though they are new pieces of meat. Because they are.
Our sex is a currency we choose to use and due to the various factors, we use it often, and often in unseemly and unsafe ways, driving a stereotype based quite in fact, not to mention the spread of disease.
This isn't to say we do not engage safely in sex, and it is true we are moving away from the cultural necessity to seek sex in unseemly places (truck stops, parks, etc) and many of us espouse "ethical sluttiness" to borrow a term.
But life is so much more complicated, and these very drives towards physical fulfillment, however fleeting, does conflict with the drive to not be alone, to find companionship and love, however conceived. It's not so much that we're doing it, it's how okay we are with doing it.