I believe gay marriage isn't very controversial anymore with the British population, and certainly not if it were to come from a party of the right.Obviously, if some of our British posters were to have the intention to correct me on that, they certainly should.
Civil partnerships effectively neutralised the issue before it could become a big deal (and the passage of civil partnerships wasn't especially rough either). The main recent political battles regarding homosexuality were a little earlier and were over the age of consent and the ghastly hangover of Section 28, and once those were won, the process of normalisation (for want of a better word) increased quite rapidly.