Loveweekends - have you state educated your DCs or been state educated yourself? did you have your DCs in an NHS hospital? Have you ever also needed any NHS treatment? Have you ever claimed child benefit?
I realised recently that once I have my second DC on the NHS (currently PG), after my state education and my pre-fees degree, my various illnesses as a child etc, even though I'm in my 30s and earning over the average wage, I still have so far taken far more out of the pot than Ive put in it. (Although if you look at DH and I as a couple, then ok, we've paid more in than we've had out so far)
This is without any major physical problems or any periods of unemployment, and not getting anywhere near state pensions or the level of care often needed from the NHS as you old.
Yes, you might be paying for someone else's pension, but then someone else paid for the GP practice you go to and the teachers at your DCs school.
Trust me, if you start pricing out the costs of going private for everything, very few people's NI contributions come anywhere near touching that...