I'm quite confused about the whole thing. It seems to be a question of moving money around for PR purposes.
As I understand it, single folks who live alone get a rebate of 25% off their council tax. Which for me would equate to £500 a year. So if you are on a low income (taking home less than £200 a week) it's quite a significant saving being single as is.
If we take it as read that tax/NI on basic income is about 30% and you earn £20K you are going to pay £6k a year in tax and NI
If you are on a higher rate you can pay about 40% (I do, it may even be more than that, my accountant would know but he refuses to talk to me this early in the morning ). If you are single and on higher rate, the saving you get by being single is frankly negligible (yes I know it's the same number but if your take home is £800 a week, then saving £500 a year seems small bananas IYSWIM).
So first point - breaks for both single and married people are intended to only help those on lower incomes and not those who earn more than £65K per year gross as a household. Frankly, once you get past that point, you are on your own and get as much off as you are willing to pay your accountant to wangle.
Second point - if you increase the taxable allowances, by a few thousand selectively (ie for those already married) they get slightly better off. Which means they can probably afford childcare if they have kids and makes getting both parents at work more likely. The sum total result of this is that there is on average more taxable income coming into the system, so there is an economic argument for doing so.
Doing so just for married people: - well there's a good argument for doing so there as well - married couples require two people to agree to make changes to their working situation so it's less likely they will. I'd give up jobs at the drop of a hat when single and childless, but now I have a nanny, career, husband etc. I'm more likely to be cautious because DH would not be chuffed with me.
Long post, so I will summarise: there is less of a total effect by reducing taxes on the affluent but overall, in total the income to the govt would be likely to rise, so it's a sensible move.
Doing it by marriage may also be sensible but it is in no way proven and morality based taxes will alienate as many as it will aid.