Personally, I've made the decision that I don't want mono relationships, so I am not keeping them as an option at all. In fact, I make it clear that it definitely isn't an option.
I think you should be telling people sooner than the third date, myself, and if you are still unsure about what you want, make that clear too. You could tell people that at the moment, you plan to have open relationships only, but that might change, and it might change with someone other than them. That means anything you have developed with them will end as a consequence of you being monogamous with someone else. They can then choose whether or not to date you under these conditions.
I appreciate the frustration you are expereincing, I went through it myself. Then I set some rules for my relationships:
No Veto rules
Nobody who hasn't already identified as non mono
Nobody who I can immediately rule out for a LTR
And, I found two great people.