All are knowledge of relationships comes from Earth and until proved otherwise will need to serve as a basis for comparison.
Regarding phisical attraction.
1.All land animals I know of have the same style sex organs for male and femals with size being the main difference and since the LI's in ME are right about the same size as a human they shouldn't be so different as to be uncomfortable or impossible.
2.Bestiality is a thing here on Earth so getting conset before hand shouldn't be a turn off to those people.
3.There is no reason biologicaly for men to be attaracted to men or for women to be attaracted to women but it does happen and their love is still just as valid as a hetro's and their is nothing wrong with that.
All that aside outside of Asari interspcies relationships seem to be a rare thing and mostly taken by Quarins on pilgramige since there are so few Quarins across the galaxy and even then they don't get phisical often.
Shepard is also a special case as the time he/she spends with aliens is often in a cramped ship under extreme stress with life and death depending on those around them so this may help to both further the emotnial connection but also soften the edge of an alien apperence as they become less turian, drell, quarian, or asari and more Garrus, Thane, Tali, or Liara.
Personally though I do think about the species when I start a romance chain depending on my playthrough and what that Shep wants. I usually play a male and when I go paragon with hope for the future I am looking for a family down the road which limit him to Ashley, Liara, Jack, and Kelly. But if I'm not looking that far forward then I do tend to lean towards Liara in 1 and Tali in 2 and 3.