I agree that if they pay for themselves then it is their business. I think I would feel differently if one person in the family paid tax and NI and the rest assumed that this would cover them all, however.
I am all for choice and people living how they wish, but not if doing so forces others to support their choices financially. My friend, for instance, has four children. She has not worked since the first was born, and her daughters are saying that they don't plan to work either, despite having had university educations - instead they will rely on their husbands' contributions to 'cover them'.
My friend's husband works and contributes, but I doubt if it is anywhere near enough to cover the cost to the rest of us of all six of them, when you consider child benefit, education, the NHS, her pension, roads, public buildings and all the other things that come out of taxation and NI.
It galls me that there are people on low incomes who are taxed at source, and effectively forced to subsidise her family, and that she would definitely not see them as equivalent to a couple (or small family) on benefits. IMO it is exactly the same, which is one reason why I think that the Universal Credit system is so unjust.
It is tricky, as it would be horrible if we had a cost/benefit analysis applied to every baby who came along, but I do feel that we need to look more closely at how our society is funded. What we then do with that information is a whole other debate, of course, but I don't feel that we should buy into the demonisation of those at the 'bottom' - there is as much subsidisation of people who identify as middle class.