The traditional institution of marriage is less changed than you might assume. Historically, it’s varied quite a bit from any given norm, the largest change is simply longer lifespans and less cheating.

The most important change in the last 50 years compared to any time before is that women now have the option to be self sufficient.

Even accepting your premise, which I don't...

Is it ethical to require half of the species to be dependent just for societal stability?

I never insinuated that half the species should be dependent for social stability. In a previous comment, I mentioned it was plausible a one woman-one man (or any 1 to 1 partnership ignoring genders) results in more stable societies due to less competition.

It lasts longer but less cheating happens? How? Has marriage actually become stronger?