There're a lot of practial differences and I can give you some:
- male/female couples might have offsprings
- Children change the amount of resources consumed and available
- States need to plan based on these numbers and their variation
- Married couples behave differently than single male and females both short and long-term which also affect governmental planning.
So there are a lot of reasons to say that's wrong to add a lot of noise to this only for the sake of calling it marriage.
Older people get married past the age of childbirth, infertile or avowed child-free opposite-sex couples marry too.
The 'signal' is already hopelessy noisy and not fit for any sort of purpose. Plus, "we were using those numbers for something they are inherently unsuitable for" is no valid reason to continue discrimination.
At this point it really looks like you're grasping at straws to justify what is just a prejudice.