Ok, let me give you a point of view by a man. Of course, this is just my personal view - and not representative for the whole male species
In a certain sense, I think both of you are right and, probably, you don't even disagree as much as it currently seems. The point is that it is not just black and white... flirting without intentions to go beyond can be a positive experience but also a negative one. But let me explain:
First situation: Imagine I'm out with friends. We sit at a table in a restaurant/bar and you pass by or you're sitting on the table next to us. We have eye contact, exchange some words, flirt a bit. Still I'm out with my friends and this is what my evening is about. Having fun with my friends. The fact that you are here and we flirt a bit is cool and makes the evening even more intriguing - but that's it. Nothing "bad" happens, if we never meet again or if we exchange numbers and nobody calls. I would have spent the evening like this anyway. I could even occasionally think about "that cool evening when I was out with my friends and flirted with this girl". So it was a positive experience in my mind. Flirting is fun - so no reason not to do it if the situation occurs.
Second situation: We've met before. We flirted, exchanged numbers and we decided to meet again. So this time, I do invest time to meet you. I will be at a place just in order to meet you - not because I'm already there anyway with my friends. But already when exchanging numbers, you knew that you would never be interested in having a relationsship with me (not even an "open" one) nor interested in having sex with me (which, by the way, would be a really bad decision
). Still you decide to meet me again. Why? Just because you enjoy the flirt itself and the attention. In contrast, I don't know about this and I actually am interested in at least having sex with you. So we meet again... and flirt again... but nothing happens because you don't want anything to happen. However, I still think you might be interested to take it one step further - because that's what flirting "usually" means. So we meet again, and so on. In this case, the flirting would turn out to be a negative experience in my mind because I actually invested (time+attention) in some interaction because of something which you KNEW you didn't want to - but actually gave the opposite impression to me in order to keep me around.
So that's the big difference... and that's why you should always be honest. There is nothing wrong about "not knowing yet" or "not being sure". That's perfectly fine. But IF you are sure about something and IF you know something - then make it clear to the other person and make sure that he does not get a wrong impression - or, even worse, purposely mislead the other person.
Now, having said this, I don't think that the last thing is what you are actually doing. Actually I guess both of you will agree with my points - and that's why I think you have some kind of a misunderstanding. According to your previous posts, you actually are indeed interested in some kind of open relationship or whatever. So flirting with somebody when getting to know the person is perfectly fine. Flirting after having stated that you want your freedom and nothing totally serious is perfectly fine. Flirting while figuring out whether the person would be a suitable candidate for an open relationship is perfectly fine. Flirting while meeting the other person on a 3rd date and still not being sure is perfectly fine. Losing interest afterwards is absolutely fine. Only then, after you lost interest, I think still flirting with the other person would not be the right thing to do because you give false hopes to the other person. And yes, in that case, I would consider it a waste of time if I was the other person.
But from what I understand, this doesn't seem to apply for you, does it?
Oh, and considering the other thing: It's partially true. German guys are definitely more passive than guys from other countries, especially if you compare it to mediterran, such as, for example, Italy. I'm not completely sure how it compares to US, but my guess would be that US guys are somewhere in between. I guess you will have to learn to be even clearer in what you say - for example "I would be interested in having an open relationship with you." instead of "I need my freedom.", which can basically mean anything and might have just totally confused your colleague. Also being a bit more active yourself can of course be a solution as well. Stating/Taking what you want instead of giving hints and hoping for the other one to give it to you can help.