Living in an effectively post-Christian country, although still de-facto Christian, I have to say an awful lot of homophobia is cultural rather than specifically religious. There is a very high proportion of people who having been brought up with with no real knowledge of the Bible or any particular domination who detest homosexuals, simply because it was what they were taught as children by people who weren't really believers but just followed the cultural norms of their time. I know many people who claim to hate gays (mostly over 50 or in the lower social-economic groupings) but cannot give any reason beyond that it was what they were taught or "it's just not natural". Trying to point out that homosexuality has been observed in over 1,500 species doesn't seem to register with these people.
Advances in gay rights have certainly been made here in the UK and civil partnerships are now recognised. However, in terms of equality one needs to look at the meaning of the word marriage. If it is to be defined legally it needs to apply to all people equally in order for the law to be just. If it is to be defined religiously then who cares what the definition is unless they are a member of said religious grouping?
Marriage needs to be defined as either an exclusively legal institution, in which case religions should have no say over who can get married beyond who can use the facilities they own, or it should be an exclusively religious institution, where upon no legal benefits are conferred on the happy couple whatsoever, unless they choose to have a separate civil partnership arrangement.
One would have thought that this would have been a no-brainier in the US with your beautiful Jeffersonian wall but it sadly seems not.
I do think within the next 20 to 30 years marriage laws in the UK will become equalised between hetro and homo and am encouraged by what I have read about Iowa having historically been a trailblazer state in terms of women's rights and manumission.
I may be wrong, but the glass does seem to be very slightly more full than empty...