I had a close male friend in grad school who'd been gang-raped 10 years earlier in the parking lot of a bar. (It was not a gay bar, my friend was straight, and there was no particular reason to assume any of the men who raped him were gay.) It was, without question, the most emotionally devastating moment in his life and a decade later, he was still struggling in various ways with the psychological aftermath of it. I don't necessarily know a lot of details about all he went through emotionally--that's not generally something one can press rape victims, male or female, to discuss--but I know enough to be very clear that he experienced all the same feelings of deep shame, suppressed rage, and profound humiliation that so many female rape victims struggle with. As far as the actual situation the rape occurred in, it took place when his family was living in another country, he was conspicuously a child of rich foreigners in a poor country with intense class divisions, and his own assessment as to "why" seemed to be that he simply made for a convenient target for some sort of collective revenge. I suppose it's also possible that there was some kind of prior altercation, or some other supposed quality they saw in him that further heightened their "revenge" fantasies or something, but if so, he never mentioned that. At any rate, at the time, he told no one--police, friends, or family--anything more than that he'd "gotten beat up" (he did have a few bruises he couldn't hide). Several years--and several suicide attempts, drug addictions, and major depressive episodes--later, he finally managed to tell a therapist what had happened.
I don't specifically recall him mentioning "masculinity" or "fear of being labeled gay" as reasons for not telling anyone, but I do remember him mentioning feeling too ashamed to speak or even make eye contact with people, fearing he wouldn't be able to handle the sorts of responses people might have if he told them, and fear of what he might do to himself or others if he allowed himself to consciously think about what had happened to him too much. I've never personally known a woman who experienced anything quite like what he did (that I know of, anyhow), but from testimonies and personal narratives I've read, I'd have to say those sound exactly like the reasons women so often give for why they didn't tell anyone, either. I don't doubt that feelings about losing one's masculinity add their own particular inflections to the trauma and present their own particular added barriers to talking about it, but in my view, the fundamental nature of the shame is likely pretty much the same. I think for pretty much everyone, male or female, there is a certain irreducible need for mastery over your own body, for control over your boundaries and who, how, when and why you allow to cross them, that must be respected in order for you to feel whole and secure in your basic integrity as a human being. And there's just something about sexual violation of those boundaries in particular that, for whatever reason, most all of us experience as being more primally degrading and debasing and humiliating than all but the most extreme forms of physical violation. I'm quite certain that most people of both sexes would far rather be beaten than violently raped.
Like MrsS I wouldn't be particularly surprised if this Houston rapist turns out to be straight, however, I wouldn't be particularly surprised if he turns out to be gay either. Finding the sexual brutalization of another person irresistibly arousing is abnormal in any case, but I can't think of any obvious logical reason why gay men would be less likely to have that trait in comparable proportions. I suppose to a point it's an interesting question why, if orientation has no essential link to it, it's so much more common for straight men to express their sadistic impulses towards women sexually than those towards men. The only connection I can think of is that straight men who rape women (serial rapists, particularly) so often seem to harbor irrational beliefs that women in general have deeply sexually wronged them somehow, that they've been humiliated and pushed around and unacceptably debased by real or imagined sexual manipulations and rejections from women, and by some sick series of dysfunctional connections, their normal sexual interest in women then winds up getting intertwined with their more general (and pathological) interest in avenging themselves on women in a drastic way. Whereas, simply because they're straight, they have no concern one way or the other for the idea of whether other men are manipulating or rejecting them sexually, making that particular series of connections that much less likely to occur. But that's all just speculation, and so far as it goes, it obviously doesn't address the "motivations" for things like prison gang rapes or, for that matter, what happened to my friend. Those kinds of crimes are extremely atypical, though--most male rape victims are raped by male acquaintances, just like most female ones--and they probably require a different explanation as to why the perpetrators in those cases (assuming they're straight) found the idea of raping another man sufficiently arousing under the circumstances to be able to carry it out to begin with...as opposed to "simply" settling for beating him to a pulp. Yes rape is primarily about domination and humiliation, but it does also require sexual arousal, so ultimately you have to have some kind of additional explanation for how that particular impulse somehow worked its way into the equation. Otherwise, the act of one man raping another would be far more common than it is.
As far as why the topic of male rape victims isn't discussed much, I think the main reason is simply that there are far fewer of them, and therefore it's a much less visible crime and much less talked about. It's always harder to talk about traumatic things that have happened to you if there isn't much sociocultural precedent out there for talking about them. As the Estrich article points out, it wasn't so long ago that female rape victims never publically talked about their experiences, either--and of course, a great many still don't, even if Victorian notions of "ruined reputations" are no longer, per se, a big issue. And *if*, as I suspect, it's the case that *most* male rape victims are gay men who were raped by other gay men (which, since it is true that most male victims know their attackers, would make sense), that could be an additional reason why this particular crime tends to remain invisible. Unfortunately, so long as so few male victims are unwilling to speak publically about their experiences and encourage others to do the same, it's pretty difficult to get much of a dialogue going about it, let alone get a good sociometric sense of precisely who this happens to, what it does to them, and how best to address both.