Well, sex with different species is hardly new in Star Trek. Their first alien EVER, was the result of Human/Vulcan mixing. Then there was Deanna Troi, Kehlar, Belanna Torres, Simon Tarses, and many others whom I can't think of right now.
In fact some other sci-fi, such as Larry Niven's Known Space series, and Babylon 5 have featured races where sexual relations is a formal part of diplomacy, (although Ivonova managed to bluff her way out of it.) I believe in the Larry Niven universe it's called Rishathra.
This has to be probably the thing that irks me the most about Star Trek in general.
Interspecies sex. That doesn't happen on Earth. It wouldn't happen in space.
Any "race" that we are biologically compatible with stops being a race, and become a small variation of the same race! (as it is here on Earth).
To add insult to injury, the Star Trek "pairings" tended to be super hot women with weird looking alien "males". DS9 took it to extremes.
I don't think the reverse was seen much at all.