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A truly alien species, compared to mammalian ones, would be seriously problematic. Especially so, for non Oxy breathers. Methane breathers or Silicon based lifeforms would really not fit in a Central Hub (ie: Citadel like). I would postulate that little common frame of references would exist.... trade being one of the few.
But, since this is a fictional universe, slapping on boobies to make them look more palatable to us humans (Quarians anyone?) isn't such a bad move. On the other hand, a human female romancing a Turian makes no sense to me. Since alien chemical/biological makeup is different from ours, body odour comes into play.... and we all, I'm sure, have met someone with that problem. 
I think, and it's just my opinion, that not every race needs to be palatable. Don't take me wrong. I love beautiful women. I love beautiful men. I love beautiful humanoid looking aliens. And I sure do understand that humanoid features make aliens look more attractive and relatable. However, I don't know why we necessarily need several very humanoid looking alien races. And I'm not sure that "It looks like it can be banged," should be a defining characteristic for any race, lol. (We already have the asari for that.) We're in space. The life forms could possibly look like nothing any human has ever seen or could imagine. (I mean, am I the only one excited about that? ALIENS!) So restricting everything to a human with a different head, or a human with different legs, etc. can be really limiting. Plus, humans are sick bastards enough to enjoy different races, too. Garrus being a proof of that, lol. (Although I've also never understood how exactly that would work, but more power to them.)
Another thing is believability. Maybe it doesn't matter to some people, but putting boobs on a creature that's basically made of metal and bones just comes across as really funny, imho. Especially if they don't breastfeed their young. Is it really necessary to make such a creature look palatable? (Imagine boobs on a salarian... Actually... please, don't... Bad idea. Abort! Abort mission!) That would be the question. We have some pretty awesome races that look nothing like us - like the krogan or yahg. I don't think that hurts anything. Heck, I want to know more about the hanar and how they deal with life with those tentacles. Etc. Aliens are too cool of a concept to be all made so similar to us. We can always have a few humanoid races and then some really bizarre ones, after all.
I agree that aliens that need different living conditions would have to be handled in some way. We do have the volus, for example. I don't think living conditions also necessarily relate to having breasts or humanoid features, though. When you look at fauna here on Earth, there are so many different species, so many different creatures, and they all breathe air. Breathing air is not really in the way of diversity, imho, although if there's any biologist around, feel free to correct me.
Then again, that's just my opinion anyway.