Many years ago I watched Flip Wilson do a comic sketch on the difference between the way a black man from the inner city spoke to a black woman and the way he would speak to a black man. To the woman he would speak in low tones, slowly and confidently; with movements to match. To the man his voice was high pitched, quick and agressive with short phrases. I remember thinking "Yep, I've seen this, and Flip nailed it."
Of course, things like this are more dramatic when seen from outside the culture. Still, if you look closely, you can see the same kinds of differences in how men act with other men and with women in mainstream culture. It's a bit harder to spot the differences in environments where everybody is taught to be aware of how they act and to be afraid of accusations of sexual harassment. Even so, watch the body language.
When women are around, men go on display. Married or single doesn't matter. Only with the very old or very young or close kin does this effect not kick in. [I assume the same thing happens with women but by definition I'm not there when women are alone together.] So long as a woman is around, the men are "strutting" and there is an atmosphere of . . . the only phrase that fits is "sexual tension." It is mild, of course--I'm not talking about heavy breathing and bedroom eyes; not talking about a mating dance; just a form of roleplaying.
Received wisdom to the contrary, I've never found any evidence that this is intrinsically bad. The details of the display vary from culture to subculture, but I see displays everywhere. When women arrive, men do the psychological equivalent of sucking in the gut and standing taller. And in some subcultures the change is quite dramatic.
When the women leave the men "relax." That doesn't mean they're glad when women leave, just that they stop the display.
You needn't remind me of situations where a woman is considered "one of the guys." Sure, that can happen; though I suspect she often isn't as much one of them as she thinks. I'm interested in the general picture here.
That general picture shows men going to the extra effort and tension of a display when women are around, and not taking on those extra roles when only men are present. It seems to me reasonable that men should properly want to spend time both with and without female company. Since this seems to be adequately backed up by observations of anthropologists (sometimes with very odd theories), I'll take it as given.
Given that men will want time with other men, what will their expectations be? During that time they expect no sexual tension; no need to display; and no worry of being evaluated on their sexual attractiveness.
This has nothing to do with "homoerotic" motives. On the contrary, introducing a homosexual into the group frustrates the expectations of the other men and can be experienced as a betrayal: betrayal of the expectation that there is no-one evaluating their attractiveness, betrayal of the expectation that "we can all relax and be secure."
Some claim that someone who reacts with displeasure to the presence of a homosexual is insecure with his own latent homosexuality. Aside from the utter lack of evidence for this claim, it never seemed very convincing psychologically. My "betrayal" model has just as much evidence to back it up, and seems to fit the character of some people I've known rather better.