Mark Oppenheimer's extensive New York Times interview with Dan Savage (who started the "It Gets Better" campaign) is titled "Married, With Infidelities":

Savage believes monogamy is right for many couples. But he believes that our discourse about it, and about sexuality more generally, is dishonest. Some people need more than one partner, he writes, just as some people need flirting, others need to be whipped, others need lovers of both sexes. We can’t help our urges, and we should not lie to our partners about them. In some marriages, talking honestly about our needs will forestall or obviate affairs; in other marriages, the conversation may lead to an affair, but with permission. In both cases, honesty is the best policy.

... “The mistake that straight people made,” Savage told me, “was imposing the monogamous expectation on men. Men were never expected to be monogamous. Men had concubines, mistresses and access to prostitutes, until everybody decided marriage had to be egalitar­ian and fairsey.” In the feminist revolution, rather than extending to women “the same latitude and license and pressure-release valve that men had always enjoyed,” we extended to men the confines women had always endured. “And it’s been a disaster for marriage.”

... There is one subculture in America that practices nonmonogamy and equality between partners: the sizable group of gay men in open, or semiopen, long-term partnerships. (A study published in 2010 found 50 percent of gay male couples in the Bay Area had sexual relationships outside their union, with their partner’s knowledge and approval.)