Oh boy, how to ask this question and remain workplace safe…
In male-male sex, I understand one person typically (ahem) “tops” and one person “bottoms”. My understanding is that while some couples may take turns, the majority have a preferred role.
But how does the “bottom” get an orgasm without stimulation of his genitals? Is it simply self-masturbation? Or is there something else going on that I’m not aware of?
Though I’m not gay, I know that the prostate can be stimulated through the rectum, and the sensation is quite pleasant.
I’m not gay, but from what I understand, people are people and some men prefer anal stimulation, some prefer oral, some manual, and some, all or none of the above.
I started a similar thread about lesbians and had my ignorance fought.
Sgt Schwartz
That’s certainly a good part of it, and why the act is in and of itself pleasurable for the bottom. In addition, the top will often be doing things with the genitals of the bottom during or after the act… R. Lee Ermey references this briefly in Full Metal Jacket 
I am versatile(both top and bottom). When I bottom I can get off in all the usual fashions. Self masturbation, masturbation by the top, oral sex before or after bottoming, top after bottoming.
Much like heterosexual sex, orgasms aren’t simultaneous or stimulated by the same sexual act. In a good match both partners work to make sure the others needs are satisfied.
I was thinking of that scene as I typed my first reply, I just didn’t want to type it out as I wasn’t sure how “work safe” it would be.
I’ll just make reference to “courtesy” 
Nature put tons of nerve endings in your butt, almost like she intended it to be multi-purpose. Beats me why, but you don’t need a prostate to enjoy it. Is someone saying it doesn’t count as “genital stimulation?”
Do note that “top” and “bottom” are generally used to refer to roles in BDSM, not to who does what sexually in vanilla sex.
“Prostate” with reference to what’s stimulated in anal sex is actually shorthand for “an erogenous nerve ganglion mounted against the ventral wall of the rectum and lying between it and the prostate gland in males.” I’m not sure about the exact placement in the average female groin, but I’ll venture a guess it’s adjacent to the dorsal wall of the vagina.
This isn’t factually accurate in regards to gay sex (which is what the OP was asking about). That is, if gay sex can ever be considered “vanilla”. But top and bottom are very commonly used as identifiers for who is doing who; no BDSM need be involved.
If you need any confirmation of this, just check the craigslist “men seeking men” section. I’m not going to link to it here for reasons that will quickly become apparent if you check it out for yourself.
Or gay.com for that matter… or really any personals site for gay men anywhere on the entire interweb 
It might be noted that a substanial number of women don’t have orgasms from vaginal intercourse either. Additional stimulation is needed.
oops missed a word in that sentence
Not true. The terms originally came to be used in BDSM terminology, but now are also used to describe “vanilla” anal sex roles. They’re also occasionally used to describe oral sex roles, but this can get very confusing.
And further, “top” and “bottom” are most commonly used in the spanking “subgenre” of BDSM. The “regular” BDSM types tend to use “Dom/Domme” and “sub” more often (you’ll very rarely hear spankos refer to themselves as “Dom” or “sub”, because we don’t think of it as Domination/submission so much as “I like to spank” and “I like to be spanked”).
I am suggesting that these are not terms unique to who puts what in whom, and thus adds to unnecessary confusion, as well as confounding the answers one might receive.
Shoshana
Here, queer, since 1975, therapist, friend, relative to many gay men who do not use the terminology as the OP did.
God you make me hot when you talk that way.
Does it then follow that if the nerve bundles are sufficiently stimulated with women receiving anal sex that they orgasm as a result? Or is that quite rare?
Cartooniverse
Ladies lack prostates. However the G-Spot (hand-drawn image of innards with labelled body parts) is supposed to do for women what the prostate does for men (i.e. enhance orgasms).
So much for workplace safe, LOL
Upon rereading this, it might be a little misleading. Though it’s probably true that most gay men have a preferred role, in reality most tend to be rather adaptable. Sure, in their own minds it seems that more than 50% self-identify with “bottom,” but in reality, especially within the context of a relationship, I think there’s much more flexibility than that would imply. Otherwise we’d have a multitude of frustrated bottoms and some very exhausted tops (yet sometimes that does seem to be the case).
And yet even for those of us who have a pronounced role preference, it does tend to be very individualistic. The terms “top” and “bottom” become virtually meaningless when considering all the possible activities that two (or more) men can engage in, including both BDSM and vanilla. Just because someone is “100% top,” doesn’t imply anything about what he actually enjoys, without further elaboration.
I have to point out here that not all “bottoms” need or desire an orgasm. And not even all “tops,” for that matter. One of the most memorable and intense and pleasurable experiences I’ve ever had was a long night spent with an amazing man, and neither of us ever had an orgasm. But I have to admit that’s rare, at least for me.