[Responding to your comment in isolation.]
As you're talking about legal things I feel a degree more pedantry is needed. You can yell "fire!" but not without an expectation of subsequent deleterious consequences. You have the freedom to speak, you don't have the freedom from consequences of that speech.
>When the target of your affection says "Sorry, I have a partner / boyfriend / girlfriend" it is a very clear signal they are not interested. //
I'd say that is "somewhat clear"; it's still relying on implicit communication. Far better IMO would be "I'm not interested in a romantic/intimate/sexual relationship with you.". Just saying you have a partner is not as clear; maybe your'e the sort that hops around, or has an open relationship or is turned on by infidelity; if a person thinks you're flirting with them but they mention a current sexual partner you might imagine that you're just being warned to keep things clandestine or being told to compete.
>most likely explanation //
I imagine the most likely explanation was that he was attracted to them, they mirrored his attraction and it went on from there without any thought from him that they were doing it in order to win/not_lose business positioning.