It's never a compliment.
I agree. I think it is demeaning and objectifying. Basically it is someone saying 'I want to have sex with you', and saying it when you are going about your business and have not asked for attention.
It can be humiliating and potentially threatening, and it is demeaning to women, as is the idea that we should be concerned when it stops. We are worth more than our attractiveness to men.
Before anyone accuses me of being po-faced, I see nothing wrong at all with mutual flirting, consensual sex between adults, or people doing whatever they like in the bedroom. It is when such attention comes unbidden that I object.