In the 'gay sex is bad unless it's women' cultural milieu, a survey by a University of Texas at Austin psychologist determined that men are more than twice as likely to continue dating a girlfriend who has cheated on them with another woman than if she cheated with another man.

The survey used 700 college students so calibrate accordingly - men may not like their girlfriends sleeping with another guy but a girlfriend sleeping with another girl is a porn movie.

But don't start yelling about young men.  While women were more likely to break off the relationship following both kinds of cheating, they really disliked a boyfriend's homosexual affair.  So, in a way, college age men are more tolerant than college age women.  At least about who can sleep around with whom.

The survey in Personality and Individual Differences painted the scenario of a committed romantic relationship (including sex) with someone lasting three months. They were then asked how they would respond to infidelity committed by their partner but some of both genders were told their partners had been unfaithful with a man and others with a woman. Some were told their partners had an affair with one person, others with more than one, some were told the infidelity happened only once, others twice.

Occurrences or numbers of partners aside, the study found that:
  • Men had a 50 percent likelihood of continuing to date a woman who had a homosexual affair but only a 22 percent likelihood of staying with her if she cheated with a man.

  • Women were more tolerant about a boyfriend sleeping with other women, 28 percent likelihood of continuing to date him, but a boyfriend who had a homosexual affair had a 79 percent likelihood of being kicked to the curb.
Lead author Jaime Confer, doctoral candidate in evolutionary psychology, and Professor Mark Cloud, of Lock Haven University bizarrely try to make a biology case that men are more distressed by infidelity that could threaten their paternity of offspring.   College age men.  Offspring.   Yeah, instead of the obvious "it's a porn movie" thing that has nothing at all to do with evolution, no matter who much evolutionary psychologists want everything about sex to be biology.

They further contend that men may also view a woman's homosexual affair as an opportunity to mate with more than one girl simultaneously.  Because evolutionary psychologists insist men have an inborn need to sleep with the planet.

"These findings are even more remarkable given that homosexuality attitude surveys show men have more negative attitudes toward homosexuality and to be less supportive of civil rights for same-sex couples than women. However, this general trend of men showing lower tolerance for homosexuality than women is reversed  
in the one fitness-enhancing situation—female homosexuality," say the authors.   

It's true.  Machismo college age men may regard gay male sex as icky but not female gay sex, as long as the women are hot.  Science has no explanation for that, try as evolutionary psychology might try to make one.  Because it isn't a biology issue.

They portray the female response stereotypically, stating that a boyfriend's homosexual affair might be regarded as a sign of dissatisfaction with the current relationship and a prelude to abandonment so they dump him first.    Whereas if he sleeps with another girl, he is just 'being a guy'.
They did ask about the outcomes of any real-life infidelity experiences and the results matched those of the imagined ones - men were significantly more likely than women to have ended their actual relationships following a partner's presumably heterosexual affair.

'Presumably' because if any of the men had girlfriends sleeping with other girls, they weren't telling.  Especially if they were allowed to 'mate' with both of them.

Citation: Jaime C. Confer, Mark D. Cloud, 'Sex differences in response to imagining a partner’s heterosexual or homosexual affair', Personality and Individual Differences, Volume 50, Issue 2, January 2011, Pages 129-134 doi:10.1016/j.paid.2010.09.007