It sounds like this was a case where each couple had boarding passes for the same seats due to an IT glitch. Couple A was already seated in the seats; couple B was not. AS apparently decided that it was couple B that was "appropriately" assigned those seats. Couple B didn't want to give up their assigned seats because they wanted to travel together.

So what it sounds like was AS had to separate and/or downgrade one couple with boarding passes with seats assigned together. I don't know how they chose which couple to downgrade, though I don't entirely buy the "possession is 90% of the law" argument that the couple seated in the seats should have been allowed to stay there.

So that is a particular situation in which it's certainly OK; they had to downgrade one couple. AS certainly should have done so with an apology and an offer of compensation: obviously a (cash) refund of any Premium Class fee paid, a comp drink in the regular economy seat, and probably also something like 5,000-20,000 miles or an equivalent discount code. Whether the downgraded couple was gay of course has nothing to do with it.

Whether or not that is actually what happened in this case, it is certainly a situation in which it is OK to downgrade someone from premium seating to reunite a couple.

Not being there and not having seen any description of what happens that gives enough detail to judge, I can't say if AS IT, the AS FA, the AS GA, the downgraded (and ultimately deboarded) couple, or more than one of the above were out of line.

All this said, the way unconscious discrimination could easily play into this is a GA or FA seeing a man and a woman with boarding passes for 3EF who want to sit together, looks at 3EF and sees two men there, and doesn't think the two men are a couple. Then the GA/FA says to the man and woman "sure, I can put you in your assigned seats to keep you together".