According to site of the French Catholic newspaper L’Homme nouveau, (link in French), Bishop de Metz-Noblat wrote his brothers bishops that the aim was not so much to be welcoming to LGBTQ+ families as to avoid possible future lawsuits for “discrimination” (which, speaking as a fellow Frenchwoman, does unfortunately not seem unlikely).
This suggested change also probably has other family configurations in view, like reconstituted families, where one of the “holders of parental authority” isn’t the biological parent of one or more children. (And at the risk of stating the obvious, some couples in such situations are in perfectly valid sacramental marriages.)
If someone is living in a union not recognized by the Church, and is not seeking to bring themselves back into a good relationship with the Church, then why would we baptize their children?
I teach ecumenically-organized baptism prep classes (mandatory for every family who wants to baptize their kid either in the Catholic or the Reformed communities of my city). It is rare that I see a married Catholic couple. In fact, the majority of the Catholic couples I meet do live in a union not recognized by the Church (and, as weird as it can sound, honestly do not understand why the Church considers they “live in sin”). They are still welcomed and the Church baptizes their children, because, as Mgr de Metz-Noblat wrote, “children cannot be held responsible for the situation of their parents”.
And, incidentally, baptism prep also offers an opportunity to explain why the Church teaches what she teaches about marriage (because some parents will inevitably ask why marriage is so important), with the result that some couples decide to regularize their situation, which would not have happened had they been refused baptism for their child.