When the first anatomically modern human looked at a woman and said, "Yeah, I'd like her to ride my baloney pony."

Being real, this is a vague, nebulous question for which there really isn't an answer. Views and expectations on social propriety have a certain ebb and flow to them. Arguably you could posit that this goes back to when humans first started to walk away from nomadic and semi-nomadic lifestyles in favor of agricultural ones- this is often argued as the point where wealth began as a concept, and as a result those who did and did not have it, and as such women become a very real asset in this time period- but this is more a question for anthropologists than historians.

There is no strict date, or record for when this first happened so, in broad strokes, you would want to analyze the surrounding context from which it comes. In itself that wont give you an exact answer, but it does give you something approaching one.