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)
All of which heavily implies that dwarf society has no concept of gender identity. Which would put offspring as the function of marriage.
same sex marriage is a concept of social rules about what genders are and what each gender does, Homophobia and gay marriage would both be incomprehensible to a culture which didn't even have a concept of gender roles and sexual identity.
Conservative/Liberal doesn't come into it, those are temporary, I like the status quo v I want to change the status quo. Whats conservative in japan would be in england, why would what is in america be in a mythic age dwarf society?
What do we know about dwarf society? it's that duty to the many outweighs personal desire, except for nobles who are fishheads when it comes to mandates. But even that shows that your role is to serve the fort not be fulfilled. I.e. It's more reasonable to assume that dwarfs would see marriage as medieval and ancient people would, as a compact to fulfil the duty to have and raise children, not as the modern west does as a personal ambition or legal contract. Dwarf Fortresses, especially in dead-civ worlds (where this is a vital issue as breeding is the ONLY way to get new dwarfs) putting sexual predisposition ahead of duty would get you as heavily thumped as it would have in the dark ages.
It's not like dwarfs are so resilient that increasing the population wouldn't be a political concern. Even the whole focus of the game is go out and increase the dwarf civilisation by establishing new settlements. And yet they'd have the entirely modern attitudes towards reproduction as to see marriage in such a way as to make gay marriage conceivable, and yet NOT have modern attitudes towards the same as to make it so unmarried dwarfs could happily have kids no problem?
And to gain what? 'inclusiveness' as forgetaboutit says. To throw aside any consideration for the setting to shove in allegorical modern politics? To go, aha, it's 2014 therefore all games must be set in 2014 in our society with our views, bugger immersion, because it's tolerant to suggest locking up anyone who doesn't share a single narrow view, but intolerant to suppose that conditions and objective reason be applied. It's not inclusive to demand that all things reflect one set of views and one set of values, that's pretty much the exact definition of exclusive.
To deny simulation to shove in allegory is excluding all other possibilities for the same of one. That's not inclusion, to include one more would be inclusion, not to replace all others with one.
But really and back to the main problem which is mechanics, it screws the viability of generation forts and eugenics. It's a bug and it shouldn't be a feature.
If dwarfs are as likely to link up in non-reproducing matches as they are otherwise, then worlds where the dwarf civs died out will just be unplayable.
If it's restricted by rare tags, maybe, it's just the game can't tell the difference when it comes to associated romances, which is what the cross-species trouble seems to heavily suggest is causing this, then it's not only a bug, but a devastating one.