At first I thought this question was actually about bestiality, and was looking forward to a hilarious flame-war. Personally I think you should be able to have sex with anything, as long as no harm is done to anyone ? physical or emotional. That is, you can wank off with your neighbors curtains if you want, as long as your neighbor consents (to avoid emotional damage to the neighbor), have sex with any being with human-level intelligence as long as he/she/it consents (again, to avoid emotional damage) or a cow as long as you don?t harm it physically.
Some might say it isn?t fair to the cow, since it can?t consent, but really, we slaughter, eat and occasionally wear cows. I can?t see how sex could be worse than that. I mean, being some human?s bovine mistress, loved and cared for doesn?t seem half bad in comparison.
That said, I hated ME2?s relationships because the asari was, from the beginning, established as some sort of sex-creatures, who had evolved in a way that let them mate with any other species. They even explained how the whole thing happened telepathically, so physicality wasn?t even an issue. Farfetched perhaps, but not more so than interstellar travel.
In ME2, however, you can hook up with anybody, and seemingly have hot, steaming monkey-sex, and they never address how! I mean, the relationships are fine, but I would have loved it if, rather than simply imply a sex scene, Bioware would have implied said sex-scene and then immediately have cut to a scene of how female Shepard and Garrus sit on the edge of her bed, and the turian explains how he thought humans procreated using spores as well? or something.
It might even have made for a great moral choice. Do you stay with the alien who loves you, but will never be able to actually have sex with you? Do you suggest an open relationship? Do you suggest adding an asari to the mix, so that both of you can get satisfaction that way? How would Garrus feel about that? How would Tali?
Anyway, that got way too long. TL;DR: Fuck whatever you want and ME2 dropped the ball.