Your surprise that UK Conservatives have embraced gay marriage explores your own ignorance of UK politics than any change in direction.
Like any political party the tories are a coalition of different philosophies, one of the strong ones being a variant of Libertarianism.
Many Conservatives simply do not see marriage and many other social issues as any business of the state, as long as no coercion is involved, so if gay people want to get married, why should I, or the state acting in my name get involved ?
Your ignorance of UK politics could be partly remedied by reading the excellent diaries of Chris Mullin, a former left wing Labour minister who as such is hardly a friend of the Conservative party, yet who describes his first hand experiences of David Cameron (FYI he's now the UK Prime Minister) as having "fine libertarian instincts".
Recall that to get to be a UK prime minister you have to be smart, a PhD in History (Gordon Brown), a degree in Chemistry from Oxford (Thatcher), Oxford academic (Wilson) or be capable of writing Nobel Prize class literature (Churchill).
Imagine any president since WWII was presented as your dentist, would any rational person open their mouths and say "drill away ?" Ask either Clinton what neo-endogenous growth theory is and they will smile and say "aw shucks" and tell you of the time they dodged non-existent bullets in a city they didn't visit. Can GW Bush even read ?
In the US being the son of a former president (Bush) or having slept with one ( Hilary Clinton) seem to be more appropriate than brains or integrity and your direct experience of a wholly corrupt system run by morons seems to have confused you.
In the UK, all but the most religious have gotten over gay marriage years ago, indeed the state Church, called (please don't be confused by this) "The Church of England#2 is almost wholly run by gay men. Part of Mr. Cameron's is to guide the Queen in choosing senior members of what is probably the largest gay organisation in the world, he has no problem with this and any number of his party are openly gay.
David Cameron is merely tidying up the loose ends left behind by Tony Blair (Oxford) and got there on merit not because of his dad, unlike the USA), and deeply religious, (hence deep friendship wth GW Bush).
The UK has never had criminal laws preventing blacks from marrying whites, so David Cameron can't repeal them (something both Democrat and Republican elected representatives fought to keep) and since slavery has ever been legal in Britain, it didn't get expressly banned until the 21st century. In contrast America locks up a scary % of its black people and makes them carry out menial work for the benefit of firms that make "donations" to US politicians of both parties.
If you weren't American, you'd know that even individual state governor elections in the US are more expensive than in the UK which has an economy and population several times that of any US state has to elect 650 MPs
The money is the reason your elected reps are hostile to things that the UK embraced a decade ago, US citizens choose to donate and support people with homophobic views. No one makes them do this, it is a free and informed choice, you can't have politicians better than the decision making ability of their voters.
I am of course unkind, you do know all this, but like other Americans you vote for people you know for a fact are wholly corrupt, there is not an American senator who if he had accepted that degree in bribes would not be in jail in many other countries.
You also seem to genuinely believe that this is a "conservative" thing, the partisan nature of politics in a country that has a higher illiteracy rate than many 3rd world states have failed to deliver you any degree of objectivity.
The short version (and I know how Americans like short and simple) is that David Cameron is a mainstream UK politician doing nothing special in UK terms because he happens to believe in it. He hasn't been bribed to do it, nor is pandering to an important electoral group because a) most UK gay people don't actually care about his marginal reform, and b) most won't vote for him whatever he did.