Here's a question that doesn't single any one group out. If all other factors were equal which marriage would you bet would last longer: the one in which monogamy is practiced or the one in which partners "sleep around?"
We could look at the numbers and see. The statistics vary in how many married people or formerly married people report they slept with somebody else while they were married. If we could compare the various reports with the rates of divorce, say, in the U.S. over the last 25 years, we might get the beginning of a picture.
The only possibility I can think of how this would relate to the question of same sex marriage is that some would consider both adultery and homosexuality to be in the same category of "sin". Again, speaking from personal experience as both a polyamorous woman in an open marriage with her husband AND as a bisexual woman, I can tell you that orientation and relationship fidelity arrangements are absolutely 100% unlike each other.
One is a natural attraction sexually and romantically toward another person. The other is a couple working together to define its own boundaries. Some polyamorous couples are fine with physical sex, but no romance with outside partners. Some limit their outside partners to just swapping with another couple. Some limit outside partners to just "courting" and emotional attachment but no sex. Some limit to kissing, heavy petting, or oral sex and no penetration. Every polyamorous couple takes the time to really explore their boundaries for what they will allow into their relationship and what they will NOT allow.
Even monogamous couples need to communicate with each other about what is allowable and what isn't. Some couples don't allow having lunch alone with somebody else that might be attractive. Some don't allow porn. Some don't allow personal emails or facebook accounts and have all forms of communication funneled through the same account to prevent any wandering around.
Just goes to show that polyamorous couples really aren't THAT different from monogamous couples overall. But, yeah, it still would be interesting to see statistics and look through a number of studies to see the dynamics of what really constitutes "fidelity."