* Could this become a "don't hire married people in their late 20s and early 30s" incentive?*

No. The only real way to avoid this is to just not hire folks who can birth or father children. You'd be mostly safe hiring folks over 45 (Though men over 45 would be more risky than women). This probably isn't a good strategy for an employer.

Marriage doesn't lend itself to children: Having sex does. Having a stable relationship does. Adoption does. On the other hand, lots of married folks don't have children, on purpose or by circumstance. There is no real way to sort folks out.