Gay marriage and infidelity: People against gay marriage often make the argument that homosexuals have a real problem with fidelity, and admitting them to the club would not be good for children or the institution of marriage in general. I don't find this to be a particularly compelling argument since heteros have nothing to brag about on this score, but one contribution I can make is too see if there is any relevant, systematic evidence to help decide if the claim is true.
The General Social Survey asked 4,964 people about their sexual orientation and, "What is your opinion about a married person having sexual relations with someone other than the marriage partner?" Here are the percentages who answered that it is always wrong:
Percent thinking that marital infidelity is always wrong
Straight females 80.6
Straight males 75.2
Gay males 58.4
Bisexual females 48.7
Bisexual males 46.7
Even straight guys do better than all the non-hetero groups.
Based on this evidence, granting homosexuals access to marriage in contemporary America is a little like a NBA basketball team that has been on a losing streak for many years who decides to add some short pudgy white guys to the roster.