Oh, don't be daft. It's never as careless as 'simply legislate equality'. Look at the UK's Civil Partnership Act 2004 if you want to see how this stuff is normally done. 264 clauses and 30 schedules to make clear what law applies to people in civil partnerships and what doesn't (https://www.legislation.gov.uk/ukpga/2004/33/contents).

When a government passes a law prohibiting discrimination in employment on the basis of (say) gender identity, it essentially inherits the previous caselaw from previous employment discrimination acts - so if you know how not to discriminate against women, you have a pretty good idea how not to discriminate against transwomen.

Finally, quite a lot of changes are made piecemeal by caselaw anyway - that's the basis of the Anglo-American common law. So even as adoption law (say) evolves, it does so casewise, not generally by handing a large edict from on high and breaking people's understanding of what the law is.

At least in the UK, parental responsibility (and, separately, custody) are well-defined, including the cases where there are zero, one, two, or more people holding it. Grandparents and friends raising children has, after all, been a thing for centuries. Ditto handling splitting partnerships, ditto handling unconventional domestic arrangements.

Ditto tenancy law. Ditto contract law. Particularly ditto shared ownership, which can even legally handle weird situations where people bought houses on behalf of other people who died, and...

I'm really not sure what basis your argument is actually on.