I'm pretty sure that other thread wasn't too long ago, though I remember who it was by and it's been awhile since they posted so maybe its longer than I thought.
I didn't answer on that thread but I will here - I would find it annoying to have somebody obviously stare or appear distracted by somebody else to a noticeable amount when I'm on a date or out and about with them one on one, but I don't mind a "hey I think that person is attractive" from established partners. I'm less comfortable with that sort of behavior during the "we are still getting to know each other and getting comfortable" phase with somebody new. If I find a guy sexy, it's about 1.5 second's worth of look to appreciate whatever I like. A female, 2 seconds. If they are seated within my sight line, I may glance more than once, but if they are walking by, I certainly wouldn't follow them with my eyes as they walked by. If I can appreciate and lust that quickly, I don't see why the rest of the human population can't too!
A party, kink event, or something where it's about meeting and mingling, I feel differently because there's more of an expectation that we would be open to meet somebody to date there. Looking a lot at somebody you find interesting while working up the courage to meet them and talk to them I'd expect a partner to do. It's when looking too long at strangers (be it me or them doing the looking, or being on the receiving end of it) then I think it can be creepy or inappropriate..