Many companies already do discriminate against people in committed relationships by making people work long hours in stressful conditions. At my last job several of the employees started having some serious relationship problems because of how demanding the company was. It's subtle enough to not be recognizable but it's there.
Not always. At the fortune 200 company I work at, they just introduced paid 8 weeks maternity/ paternity leave that can be taken in any form over the following year. All the engineers think it's great(ok the ones past prime childbirth age are a little jealous) , and a VP (a man) just took a substancial leave after the birth of his kid.
It's all about the culture. Get the culture right and the decrimination goes away.
Perhaps increasing the scope of the worse hire will make discrimination impractical, but it is still going on while it's at roughly 50%.
You can somehow make companies unable to discriminate by relationship status, and that would push it into "age bracket" discrimation.
I believe the problem is more fundamental and a cultural solution is required.