At what point is the boundary crossed with a married coworker?

So today I found out that a guy at work who had been flirting with me over several months is married! I had no idea.

So my question is at what point is the line crossed? Nothing actually happened, but does that make it ok?

I know married people still look, but I don’t know how I’d feel about my husband making eyes/checking out consistently and engaging in flirtatious behaviour regularly with the same woman behind my back.

Why wouldn’t he have said?

He always kept her out of the picture even when making small talk about what he did on the weekend etc.

why would he seek my attention?

At what point is the boundary crossed with a married coworker?
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