It's not a question of taking sides, what they're doing is, basically, excluding 10% of the population. I don't agree with your assumption that it would be the start of a slippery slope and have animal activists and the like knocking on their door. This is a question of basic human rights. You honestly think it will take 6-12 months of development time just to add same sex relationships? How long has the entire game itself been in development for? It would be a minor tweak.
If I do end up getting this game I'll be wondering wtf would I want my Mii to marry a female character. I most likely wouldn't bother, and would miss out on all that exclusive content.