One reason, besides the obvious fear of a culture war with the conservatives about sacred family values, maybe the fact that the government already pays four attempts of medically assisted procreation to heterosexual couples and doesn't wish to have to extend this costly favor to gays, nor, more importantly, does it want to change at any price the national moral doctrine.

CNN: Gay rights: How U.S. leads the way

Procreation and socialisation of children within the family provide the sole justification for the institution's unique status in society.

ECONOMIST: Gay marriage, debated