Someone mentioned word choice, and I think it deserves an entry all its own. Word choice makes a huge impact on the feel of a story. The wrong word in the wrong place can bump a reader right out of the universe you've created for them, and that's a Bad Thing. You've got to keep the mood; most likely your reader is a woman, reading about two men getting it on. She may be imagining she's watching, or one of the participants, or whatever; being shaken out of the story is a big let-down. Similar things can happen with spelling and grammar mistakes (and I've seen some hilarious ones, but maybe that's another entry yet), but using spell-check and a beta reader is a given. So is getting your anatomical facts straight (as it were). The best known site for this is http://www.squidge.org/~minotaur/classic/eroc.html
It is your friend. Really. Most of your female readers may not know if you're getting it wrong, but many will, and they'll find it amusing. As will any male readers you have. That doesn't mean you can't take some poetic license. You don't have to write it like a textbook, and you can certainly skim some of the details. This is romance, after all. But as long as *you* know what's possible, you won't make mistakes.
With that out of the way...
Besides choosing words that sound sexy, appealing, etc., you also have to keep in mind to choose words that make the action clear. That's a special challenge when both your characters are men. You have to use a delicate balance between names and pronouns. For example, something like, "John smiled at Paul. John slid John's fingers slowly over Paul's thigh under the table," would seem awkward, but so would, "He smiled at him. He slid his fingers slowly over his thigh under the table." You know who's touching who, but your reader can only read the words on the screen, not your mind. It's hard to know when you have the right balance, but leaving a story, then coming back to read it a few days later might help. Or having someone else read it.
Euphemisms, especially for the male anatomy, are countless. Some of them are good, and opinion varies on that. I find penis, dick, prick, etc, unpleasant, but not everyone does. Cock, arousal, erection, length... I feel these strike a nice balance between clinical and silly. Some of the euphemisms are giggle-inducing. Man-meat, rod, tool... the list goes on. You don't want to use the same word all the time, because when you are dealing with M/M, there are an awful lot of cocks to deal with. But there are plenty of alternatives.
In conclusion, the words you use will either pull a reader into your story, the world you've created, or not. The choice of words is up to you, and no two readers are alike. But if you find what you've written to be sexy, there is a much greater chance your reader will!