There isn’t anything to correct, and that’s why it’s just highlighting it, for the human to look at it because it is a statistical use consideration. If it was used not-infrequently then it wouldn’t have been triggered. Isn’t that how basic non-AI grammar and dictionaries work, and how actual dictionaries consider adding and removing words each year? Archaic and evolving use is still valid if it’s understandable in its context. I get what you’re saying, we should use such things in our text to convey our own thoughts, and thereby make it more statistically valid in the future, but this isn’t the machine trying to imply gay couples don’t exist. Again, there’s no autocorrect here to prevent the human from just looking at it and saying, no, that’s correct.

The real question from a technical pov here is what kind of usage statistic does it take before a conventional grammar checker swings away from flagging something as unusual? I’d think it would have to be rather low to avoid lots of markings. And what sources are being used to evaluate this, and how often?