I don't have any great "tried and true" suggestions.
I hear your frustration.
It is different for me, because I'm a woman, and right or wrong, there is a different attitude then for a man.
I can't really understand the dating/conversation about poly thing. In 4 years, I haven't met anyone that I engaged in conversation with for more than 5 minutes, who didn't hear about my husband and my boyfriend (and kids and grand kids-usually in exactly backwards order). That alone almost always results in them asking "you have a boyfriend and a husband?" which in turn results in poly coming up in the conversation.
That in and of itself insures that people know very well that I'm not mono-before the possibility of me being interested in them ever comes up.
I also have it posted on my facebook profile and other online profiles.
So, I don't really know how one would get through to that point if it wasn't that.... "out" in their life. We all live together, so it would be really hard for anyone to get to know any one of us without knowing. All of the guys coworkers know, same reason, it's impossible for it to not be obvious without great effort to conceal it. (I don't work, I'm in school, so I don't have coworkers).
I'm sorry that you are struggling and frustrated.