I have to agree with those that say it is something that needs to be disclosed before the first date.
The amount of time that I have to get out and go on dates (particularly first dates) is really limited. I do NOT want to go on a first date THEN find out that the person I am on a date with is actually not a prospect for a relationship. It would be a waste of what I consider valuable time. If I am not interested in an open relationship, then a first date with someone IN an open relationship = a waste. If I am ONLY interested in an open relationship, a first date with a monogamous only person = a waste. Sure, it could be fun, but ultimately there's no point unless we're both interested in being friends. And let's face it, if you're meeting someone online with the intention to date, feeling like important information about that was hidden isn't going to be conducive to a friendship either.