Fast Jimmy wrote...
Disgarding the whole premise that alien sexual organs could very well be in anatomically respective places for sex to even be completely impossible, let alone enjoyable (or even survivable... given the dextro-based biology of both Turians and Quarians, their... juices... for lack of a better word... may dissolve human tissue after rigorous rubbing...), I suppose nothing is outside the realm of possibility. People commit beastility and, even though that is a crime, I suppose it has more to do with animal cruelty than anything else. If you had a consenting member of another species, I don't know why it wouldn't be something that happened... barring the above logistical and biological considerations, of course.
You need to remember that humans have all sorts of benign bacteria and other organic mechanisms to fight off infection. If other aliens have the same, you might literally die from some alien bacteria located in Tali's nether-regions.
Again, I think most people are primarily attracted to the aliens' personalities and are not looking at it from a desire perspective, where they want to have sex with aliens because of their weird or different biology. Plus, even though Tali and Garrus are obviously not human, they still have physical aspects that some find attractive by human analogue. I've heard many females say that they like Garrus' broad chest and shoulders, and Tali obviously has feminine breasts and hips. I mean even if you're not specifically attracted to alien anatomy, it falls into the "works well enough" category.
To indulge your biological curiosity: dextro and levo DNA species' microorganisms would, on a base level, be benign to each other. Viruses attack DNA or RNA, and since these are incompatible across DNA types, they would be completely ineffective. Bacteria and parasites survive by feeding off a host, but they too need to consume specific amino acids. A bacterium that eats levo-amino acids to build it's DNA and proteins couldn't survive inside a dextro DNA organism, because there would be no food.
The danger, as the games explain, comes primarily in the form of acute allergic reaction. Your body may incite inflammation around foreign tissue simply because it is foreign, even if it in itself is benign: see pollen allergies, for instance. This is the primary way in which Quarians get sick, and in which humans or Turians could possibly get sick. It is not the foreign matter that is causing harm, but your body actually harming itself through an overreaction in its own immune system.