Don’t confuse correlation and causation.
It is a lot more likely that the same changes in attitudes that promoted the Roe decision also promoted premarital sex and out of wedlock births.
Not to mention the colossal expansion of the welfare state.
So the same set of attitudes that led to liberalized abortion laws and to Roe vs. Wade also led to more liberalized attitudes about sex outside marriage, and more liberalized attitudes about out of wedlock child bearing too.
And at the heart of such attitudes is the idea that we shouldn’t make any “value” judgements. By definition you would have to make value judgements about behavior if you want to say no to abortion, no to out of wedlock parenting, no to the concept of sex with no strings attached, etc.
But it was Roe that legitimized these attitudes. It may not have directly led to the out of wedlock births but it was the catalyst that gave them their jump start.