Given how hard and slow breeding dwarves is already, it's hardly 'happy'
On worlds with no surviving dwarf civilisations, so no migrants, but lots of lovely reclaimable fortresses, it'd pretty much mean no game if you dwarfs didn't couple up in breeding pairs. You need alot of luck to get couples as it is without adding in the chance they'd couple the wrong way.
It also doesn't make a vast amount of sense for dwarf culture - which so far has been established as fairly conservative.
Is it really though? As far as I can tell there's already pretty much perfect gender equality. All jobs are doable by both sexes with no difference inherently in quality (of course), and as far as I can tell there aren't even masculine and feminine distinctions in names. I mean, this isn't directly related to same sex marriage in dwarf culture but it's just an example of an area where there isn't any discrimination or social rules like that.
(Really hoping this thread doesn't get super political, I'm just talking about the game)
I've always thought, that had to do with the dwarves' fairly long lifespan. The [MAXAGE:150:170], coupled with child mortality apparently not being a thing (yet), takes off a massive burden on dwarven women.
Something similar happens with Almea's
Elcari. Though they do have gender roles, both genders are considered about equally (the roles themselves are only based on practical concerns), and as a further note, don't even bother blinking at gay dwarves as long as they're loyal to the community.