Huh? I suppose there have been left-wing style social justice advocates, nominally of the church, who've advocated such a thing, but the true Catholic position -- and this is the actual meaning of social justice -- is to obtain critical i.e. food and housing aid to the poor from the most local level first and then move up if absolutely necessary. For example: family, church, employer if any, charity...and if all those fail then government might be the next step but only for essentials, not frills.
Why should I pay to support kids who resulted from their ban on contraception?
Well, in truly needy cases -- and where the local effort has failed -- I believe in helping out children regardless of what their parents did or did not do. This would seem to be consistent with tenets of Christianity.