I think am in support of the whole same sex marriage thing if only they can they can produce birth without adoption or artificial sperm insemination if they can't then they should be given the opportunity to do so as well.
A lot of countries provide subsidized or free artificial insemination for childless couples. If you consider gay couples as equal to heterosexual couples, then they will also become eligible for these subsidies. And since none of them can have children naturally, everyone will go for it and this will bankrupt the healthcare system.