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.
What you won't find is engineers, of any kind, developers, or any kind of practical education.
Which means laws very often ignore practical considerations as to how they were applied. An example often given is equality between men and women. Since 1980s and up to 2005 in various countries laws were passed that married and unmarried couples are equal before the law. Including of course same sex couples, or "couples" involving more than 2 people. I'm not complaining about that concept, but it's implementation is emblematic of just how bad governments are.
However ...
1) Okay, they're equal. So when are 2 or more people a couple ? Not specified, of course (courts seem to have settled on when you share address for a given amount of time you're married. So Good News (tm) ! You were legally married if you had a roommate in college).
2) Related. When are people not a couple anymore ? Not specified.
3) What if these people separate ? Are the any obligations ? Not specified.
4) What if people are living together and one, unbeknownst to the other, incurs a great debt ? Can that debt be held against the other (like for married couples) ? Unspecified.
5) What about shared ownership ? What about registering a property/house in both their names ? Unspecified.
6) Does entering in a couple relationship (as in living at the same address for a while) give raise to obligations in case of separation ? What about kids, adopted or otherwise ? Does one party owe the other alimony if they separate in less than amicable circumstances ? Unspecified.
7) What about all of tenancy law, family law, contract law, ... all of which applies ("applied") differently to couples and separate individuals ? A million questions. All of which are unspecified. What if 3 people are living together and one adopts a kid ? Do all 3 have custody now ? How about separation ?
Congress, however, merely wanted to look good voting a very generic law into existence. There was close to zero consideration of how that affected all existing systems, and even when problems became apparent, no reaction from parliament.
And if it's this bad for things that are directly in the domain of lawyers themselves, you can imagine just how bad it is for things they don't understand.