Three Answers:
The word ‘marriage’ is semantics they already had all the benefits and ‘rights’
It was used for exclusively for traditional relationship for eons.
What did they need to have that word for?
If they so despise that word let them push to completely abolish the concept of marriage altogether for all!
It presupposes a sinister nefarious agenda that they spent billions of $ ,focused careers took over the western world all in order to ‘hijack’ and subvert a word!
Second,All societies from the beginning of human existence had a varied of personal styles
The Greeks, for example, were quite deviant. Many of them disliked marriage and disliked women.
YET, all of them ,every single society from the aztecs to the kalahari bushmen. From the Siberian tartars to polynesians
In every era every century , every clime -GREEKS Included-Have remarkably always understood till now that marraige could only mean between opposite genders