Okay so a bit of correction :
Homosexual marriage is not blocked. Hell, a couple can be homosexually married because they don't want children while being in a heterosexual affair. It is exeedingly rare but it happens in game. Marriage is the only way to have children currently, but as far as I know it is temporary and out of wedlock kids are part of the plans.
As for homosexuality in the middle ages
it is one of the big myths about sexuality. Tolerance of gay sex, even male gay sex, was context dependant. Sure you could be accused of sodomy by the church and burnt to the stake, but if you're Phillipe Le Bon and have enough troops to your disposal to single handedly end the 100 years war, the university and the papacy just tends to make excuses and look the other way. Even notable, historically confirmed and condamned "sodomites" like Jeanne d'Arc's companion Gilles de Rais was only judged after he became an inconvenience for his family for unrelated gambling addiction issues. That's the big things in the middle age : there is no magna corpus, no universal law, might usually makes right and people will find excuses for the powerful and reasons to bully the weak
As for "church approved marriage" : it doesn't exist in the middle age. Marriage is a contract between two noble families and has abslutely no need to be sanctified. Adding a priest to a wedding ceremony is a modern fancy. The word of god, you see, was never really an absolute thing nor something that really enters into consideration when taking actual decisions that affect reality - and was certainly not an open door for clergymen to put their nose in a political contract between noblemen.
As for robes, they were male garments for most civilizations until the development of equitation. They remained gender neutral until recently. Clergymen still wear robes to this day. Robes are easier to make, maintain and replace than pants - with the caveat that they cannot be used on horse - so it makes sense that lowtech civilizations that use horsecarts instead of riding them would use robes instead of pants as a gender neutral garment.
As for transdwarves... I just don't understand how they would fit. Like, DF's dwarves are free to love and marry whoever they want. I'd go as far as to say, within the context of a developped magic system, I would not be opposed to gender-swapping options, but without that context, with no genderlocked garments and no gender roles, transitionning look like a bit without object....Or am I wrong somewhere?