The problem with the Conservative attitude to marriage is that they assume it reflects their values, and their values alone.
In a manner that harks back to the 1950's, Cameron et al like to refer to marriage like some bygone security blanket, everything was better back then when people got married and stayed together.
But, as advised by jim 360, it's not 1954 any more, even though the Tories wish it was, and like to pretend it can be again be offering an imagined incentive for people to get married.
If I really thought that the end result of this cynical carrot waving was that people got married for the allowance, how deeply flawed a society would we have then? People who married not from love and commtiment, but because the Conservative government gave them some money - talk about buying your voters!
If people choose to get married, they get married - it has nothing to do with wanting the approval of a bunch of anally retentive public school boys who think that society's values can be bought like this.
Similarly, anyone not inclined to marry for their own personal reasons (come on Dave, you know about personal reasons don't you - it's why your MP's fiddle expenses and perjure themselves fairly regularly) is unlikely to be impressed by some wooly-headed notion that if everyone got married, the country would be wonderful over night.
You don't have to beft-wing to find this policy odious in the extreme - just possessed of some moral fibre and a mind of your own.