Hmmm, this is the easiest and the hardest thing to deal with in M/M writing.
Easy because everyone has sex (okay... not everyone, but a REAL good percentage of the world). The emotions, whether you're male or female, and whether you're playing with someone of the same or opposite sex are generally the same. Fear of rejection, longing, lust, all of those are basic human emotions. Unless you're writing way out there space aliens those emotions will play a large part in your writing. If they don't, it will sound like a stroke piece from the Hustler letters page (which pay pretty good). But, what makes a romance a romance is the emotional roller coaster you put the characters on. If you write fairly well, you should be able to master the emotional aspect of M/M writing.
Hard, in more ways than one, because if you're not a gay man, you're really not familiar with the whole mechanics of the thing. Unless you've engaged in anal sex... then you've got the bottom half of the guy on guy act down (or if you're a open minded guy, who's okay with toys). But homosexual sex, like heterosexual sex, is not always an insert tab A into slot B proposition (we'll deal with that on a another day).
The one must have, in any story, LUBRICATION. Outside of the Yaoi Genre, men's asses do not self lubricate. Unless they are flat out drunk or really experienced (most likely a combination of both) you're characters are going to need some help. And even then, it may not be a pleasant experience for the inserting party -- the friction can be a little too harsh.
Examples of Lubrication:
Spit Pro: you always have it around; Con: there really just isn't enough
Cum Pro: fairly readily available; Con: Not such a good idea in the era of Aids
Baby Oil Pro: Cheap, readily available; Con: Not good with condoms (and if you're doing moderns you'll have to deal with the issue of condoms we'll take up making them sexy later), messy, hard to carry around with you.
commercial Lubes Pro: Not too expensive, good availability, comes in convenient individual sample sizes Con: you don't always have it on hand when you need it. And that leads us to another topic (we'll hit later) what can the guys do if they're out of glide?