I find at bit amazing that interracial relationships is something that is out of the ordinary. For me it's perfectly normal. Lot of friends of mine are in a relationship were the other person has "different roots", my own relationship included. My husband and I even joked about the fact that our Surinam and Dutch Antilian friends (the Afro-Dutch so to speak) all are married to a Dutch woman with long blond hair. Interracial sex is seen as part of the interracial relationship, so normal.
But it's the response of other people that make you realize for some people it's not normal. Like some people when they see us together or at the last school of my daughter when she was being teased about "being different."
When I met my husband - who has more roots he can count and like he tells me "always the only colored kid in school"- and he introduced me to Star Trek, the first thing he said "It's is the show with the first interracial kiss. It meant so much to me. It means interracial relationships are normal." For him it was a great deal.
But that was the sixties. Today, I do like shows like doctor Who who had Rose and Donna in an interracial relationship without making any fuss about it. The show isn't like "o shocking" we have an interracial relationship - it's normal.
Maybe there is a difference between European shows and American shows in this respect - I don't know.
As for sex on the show: the shower scene was one of the best love scene I have seen

(but I am biased).
But as usual sex and relationships aren't shown very realistic, for ST I think DS9 was the great exception.
There are always cultural elements in Star Trek that you have to take in account. I always joke that in American shows they showed the most bloody scenes in crime shows and that's okay (while the European crime shows are more about the psyche of the killer - hardly any blood is shown), but showing "nature" by showing a naked back or worse butt is terrible.
Nudity also seems always to be connected with sex and I think it hasn't has to be.
(I am not saying this to jugde a country, it's more a observation. Dutch people are mostly very private and conservative to speak about their sex life - they don't - except maybe with their spouse. But if some one would walk by half naked - they are okay with that. Each their own.)
Just my two cents on the subject and hope it was somewhat interesting.

EDIT: O boy. Seems like I was OT. Cultures and cultural difference are just my thing, so I get carried away a bit.