1. There's a lot of material on this topic, but Wikipedia gives a
decent overview and has some quotes. For example, there's an off-hand remark in a play, "No one prohibits anyone from going down the public way (publica via); as long as you do not make a path through posted land, as long as you hold off from brides, single women, maidens, the youth and free boys, love whatever you want." The rest of the article has lots of sources as well, particularly on slaves who were assumed to be property of the master. Penetrating them orally or anally was the right of the master and incurred no shame for either, so "pollution" was not used.
The term "mutual pollution" or "mutui stupri" is interesting. Stupri means assault or rape, which then gets us mutual rape. But if both partners wanted to be penetrated, how is it rape? Clearly, the Romans did not consider rape to be a consensual or nonconsensual according to the individuals but along class (free or slave) lines. You couldn't possibly rape your slave. And you couldn't NOT rape a free citizen, even if he wanted it.
2. Eh, there are no shoulds. The g0ys are quite against anal. Personally, not too keen to get fucked, but what does that matter? Physiologically, the prostate likes to be poked. In any case, there's plenty of non-penetrative sex as well. I would like to say that gay men like to be penetrated more, but that could very well be that most men are afraid of the feminine/penetration stigma and association.
3. Sure, I did a video that may be of interest:
http://www.youtube.com/watch?v=3EtSeWQvlIE
