It has to be brought up before the first date. I'd probably also discuss it on the first date just to make sure we're both on the same page. Why waste anybodies time and get them out without letting them know beforehand exactly what the situation was?
Who says it's a waste? You're assuming things if you think it's automatically going to be a waste.
Perhaps they date multiple people. That's more probable than not in my area, if you're on a dating site. Perhaps they're polyamorous. That's a little less likely, but ups the overall probability. Then you've got the people who might run screaming if they
see polyamorous on the profile, but who can handle it once you talk about it in person. That's a huge amount of people.
By no means, in my opinion (I'm not stating my opinion in absolutes, they're are opinions, for goodness' sake), would it be dishonest, based on the way people I know date.
Why would
anyone expect to show up on a first date and be able to compel you to be monogamous from the start? That's something that has to be discussed. I find it rather funny that on a polyamory forum like this, giving into the idea that monogamy should be expected and the default is being purported as the norm.
I don't see this as dishonest if
the person reading the profile is operating off their own assumptions, and you're not refusing to answer questions when you meet.
I will say that the stigma of polyamory is enough to separate two people who might have otherwise met. Everyone has to judge for themselves whether it's a waste of time, because of failed dates, not to put it on their profile.