Christianity does not universally prohibit gay marriage. There is actually a wide range of reactions to gay people in each of the sects perhaps most surprisingly for me:
Old Catholic, Allows as member, Ordains (after 3 years?), and Blesses unions, but does not allow Marriage.
On the other hand Episcopal (2,057,292 members) expressly allows it as do other sects.
Many fall somewhere in the middle and are not unified in there opposition, Anglican (80 million members), Pentecostal (115 million followers worldwide in 2000), and Lutheran have somewhat mixed views with some local denominations allowing gay marriage while others apparently prohibit it.
See: http://en.wikipedia.org/wiki/List_of_Christian_denominational_positions_on_homosexuality
PS: You see something of a fractal pattern on the issue ex: The Evangelical Lutheran Church in America, the largest Lutheran church body in the United States, as of August 21, 2009, voted 559 to 451 in favor of allowing non-celibate gays to become ordained ministers. The Lutheran Church - Missouri Synod (LC-MS), the second largest Lutheran church in the United States at 2.4 million members, does not ordain homosexuals. Lutheran churches in Germany are also divided on the issue of blessing same-sex unions. In general, very few churches in the more rural parishes (Baden, Saxonia, Hesse-Waldeck) are in favor of blessing same-sex unions while the urban churches do allow them (Hanover, Rhineland, Westfalia, Brunswick, Oldenburg, Berlin-Brandenburg, Bremen, Northelbia...).