For those who doubt that gay men in committed relationships are much more promiscuous than their heterosexual counterparts, read this article at The Daily Beast.
Obviously not every gay couple has an open relationship, but the ideal of monogamy is not present in gay relationships to the same degree it is in heterosexual relationships.
This makes sense. Most men are not naturally monogamous. They would prefer to have more than one sexual partner at a time. The reason most men resist this desire is because the stability of their preferred relationship depends on it (i.e. their favored woman demands it as a prerequisite to continue the relationship) and/or because of their religious beliefs about the sanctity of sex and marriage. Many gay men do not subscribe to traditional moral/religious sexual ethics (which not only proscribe sex outside of marriage, but also proscribe gay sex), and there is no female in the relationship to demand monogamy. In other words, the traditional restraints for monogamy are removed in gay, male relationships. It’s not surprising, then, that gay male relationships (unlike gay female relationships and heterosexual relationships) are often not monogamous, but include outside sexual liaisons.