>The only known tax benefit is tax deductions for kids, i.e. one may claim them without being legally married anyway, but it would be a problem to make kids in same-sex situation. I wonder if someone may consider it discriminatory.

Not really. IVF is available for homosexuals. Whether they need a semen or egg donor does not make a difference from a legal pov.
I had to deal with a few of those cases in my software.