Perhaps a more refined approach is for the child-free by choice to contribute at a higher rate since they are not providing replacement tax payers.
What if they already are contributing at a higher rate?
For example, are there more stay at home parents than childless people? As a childless person with unrestricted earning potential, I already contribute FAR more to SS than my SIL who is a SAHM of four - who contributes nothing monetarily, and never will. She has contributed valuable taxpayers (of course, some of them may also opt out of the workforce) - but at a discounted rate, due to the time value of money. After all, I am paying taxes now and her children will pay twenty years from now.
Meanwhile, my contribution to SS exceeds even that of average working parents, since my childlessness has translated to increased earnings and job creation (my husband and I were able to dedicate time to a successful start up).
So what if - when we shake out all of that math, a childless person is already contributing more than their "fair share" than a stay at home parent? Maybe SAHPs should also be excluded from the SS system.
It doesn't really matter - Social Security is designed to be a safety net. No one should be excluded, because it violates the spirit of a safety net. The area for change in Social Security will probably end up being on the back end - the max rate for taxing benefits should probably be 100% instead of 85% and all non-taxable income (including Roth distributions and municipal bond interest) should be included in the calculation for how much of a person's SS gets taxed.