it would be interesting but would a relationship between a nord and argonian or a khajiit be considered bestiality and looked down upon by the general population of skyrim?
I'm going to say
no it would not be bestiality in any form. Humans are ape-like. Argonians are lizard-like. The Khajiit are cat-like. Even in the form that walks on all fours, the Khajiit are still sentient people. So no not at all, since they are still people in the Elder Scrolls world. It's more akin to human-Klingon relations in Star Trek, etc.
Fallout: New Vegas did a great job with homosixual NPCs. But TES being based on the Dark Age era, and Medieval societies, homosixualism and such would be punishable to death.
So it's unlikely that you would see NPCs expressing this, and even if they were homosixuals, they would be scared of telling someone, because they could tell the law, and consequently those would be executed.
As has been said this isn't Earth. Secondly even if it was this reminds me of a Deep Space Nine episode. Captain Sisko (who is black) doesn't want to go to a holoprogram to have fun, because it takes place in an early 20th Century casino, a place black people would never have been welcomed equally. His girlfriend replies that even if that was the case, the program represents how it
should have been. That holo-program in the show ignored the historical racism in the interests of presenting fun for the characters. Just as for TES, it doesn't need to throw homosixuality in your face, but that doesn't mean it shouldn't exist just because such times were extremely bigoted to anyone slightly different back then.
There are references to homosixuality and bisixuality in TES as others have said. There's the diary you recover in a Fighters Guild quest in Oblivion for example where the guy is talking about his "beautiful male" companion as well, if I missed someone mentioning it. There's also the http://www.uesp.net/wiki/Oblivion:Ahzirr_Traajijazeri that includes the line:
"Life is short. If you have not made love recently, please, put down this book, and take care of that with all haste. Find a wanton lass or a frisky lad, or several, in whatever combination your wise loins direct, and do not under any circumstances play hard to get. Our struggle against the colossal forces of oppression can wait."
That skips the whole homo/bi/heterosixual debate and jumps directly into the territory of group "activity."
I wouldn't mind if it's how it was in Oblivion and Morrowind. It's acknowledged in both, if not in your face. I wouldn't mind gay, bisixual, or inter-species relationships being encountered in Skyrim though. They just don't need to draw extra attention to it. They can simply exist.