according to the priests of mara, marriage doesn't actually mean the same thing in skyrim as it does in real life.
it's more of a partnership to better the interests and survivability of the two individuals
(although it does lead to a lot of "i love you" conversation messages from the npc - which i think probably comes from bad communication in the writing department about what they wanted it to mean)
personally i see the marriage in skyrim as a means of forming a 2-person adventuring party. you play well as a ranger but need a meatshield/tank, so you marry them and you go down in the annuls of history as the unbeatable adventuring duo.
but yes i do agree that it's bad form that they made so many people in the game cool with same-six partnerships, whilst having (as far as i've seen so far) none of the NPCs in same-six partnerships themselves.
and i agree that it's odd that they included a fair amount of species-ism/rascism but no homophobia in the game. however the easiest way to answer that is thus:
beth don't want to get sued.
if you look at the game's rascism, none of it can be applicable to real life. there's no khajiit or argonians on Earth, and neither are there any elves or imperials or bretons. thus none of the rascism in skyrim could really be seen as an attack on any group of people in real life. whereas, homophobia is something that DOES exist in real life, and is an issue that delving into in a game is like opening a can of worms. Game companies are quite careful about stepping around issues such as this in their games for the most part. (ok you DO occasionally see contraversial issues in games, but they're the exceptions that prove the rule rather than anything else).