There is no homosexuality in the Dominion!
Well actually there wouldn't be. Changelings are asexual and who the heck knows how they reproduce but my money is on the amoeba analogy. Vorta and Jems are deliberately engineered to have no reproductive ability, since that would be a challenge to the Dominion monopoly on their creation via cloning.
As for Dominion subject races, who cares what they get up to as long as they pay their taxes.
Someone above made a good point. Spartan culture (if I'm not mistaken) and all of pre-Hellenistic greek culture had a strong homosexual element. I don't see Klingons as so far removed from a warrior ethos that would encourage a closeness where lovers would fight fiercely for one another in battle.
The fact that we've seen female Klingon warriors throws some cold water on that notion - the whole battle-bonding thing could be heterosexual (which wasn't an option for the Spartans) tho perhaps not exclusively. I can see Klingons having the attitude that homosexuality is just part of life, who cares anyway, it's more important to obsess about honor and sing drunkenly for hours on end.
Anyway, Trek has
depicted homosexuality - sadly, it's been pretty juvenile, just the leather-lesbo cliche in the Mirror Universe (the Intendant, Mirror Ezri).