The biology of all the ME species didn't make sense. Not the Salarians, not the Krogan, not the Asari and most of all, not the Quarians. There was absolutely not reason for their immune system to change at all in the 300 years since they left their planet.
They used to be a council species before, they lived on the Citadel, various colonies and among aliens. They had aliens on Rannoch. How was that no problem with their biology? What changed later? Even ignoring the life ships, sterile air doesn't really exist if you have thousands of people on board.
Okay, I romanced Tali once, too... but I still think that cross species romances are unrealistic, even within a sci-fi universe. Especially if one partner can poison the other with their bodily fluids.
I guess all we could do was wait for the newly updated codex. Honestly, I don't really expect games like these to be consistent with actual science particularly from people who prefer to write Star Wars and so science fantasy that was loosely based on science is tolerable enough.
Alien-Human romances aren't new. I think what make Mass Effect significant was how you're able to initiate it yourself and that it is an option (despite Mac Walters' attempt at strongly suggesting that Liara should be your canon romance). I guess the idea isn't how realistic it was, but how it was able to satisfy player's fantasies... they tested the waters with Liara (but the Asari still have a human female body and feminine features than alien) and then they comply by making Tali and Garrus romanceable and creating Thane, an alien with a sexy male body.
For me, I don't think either Asari or Quarian female's sex organs was functionally the same as human female. I remember Mordin mention how Drell was venomous and Turian semen could cause severe rash and irritation (?!). So, I assume everything was more heavy petting and otherworldly coitus experience than actual sexual intercourse.