I thought he was bisixual.
That does seem to be the case.
Well, sorta. There are female Bouyant Armigers, and Vivec is both genders, so...
Women can be attracted to their own six too.
and the beast races tend to keep to their own kind as well; I know of no Khajiit-Argonian couples, nor have I heard of any Khajiit or Argonians seeking out an Orc, Elf, or Human for relations.
Barenzhiah has learned the hard way that Khajiit can be interested in other races too, and I'm sure that wasn't the only time in history it's happened, although it probably isn't too common, the barbs likely serve as an effective deterrent when the Khajiit are male. As to Argonians, I'm not sure, if they're like real lizards, I don't think soft skins would have much interest in them for the most part, and the reverse of this is likely also true, of course, given how real people can be, there are definately exceptions.
Most likely, Bethesda just chooses not to deal with the matter for the most part out of fear that people might find it disturbing. It also might be that the people who do it tend to keep quiet about it, since racial prejudices might cause the public to look down on them, of course, would YOU talk about it if you slept with a humanoid lizard?