Lemmy.world is having some serious issues so don't know if this will go through.
For me, it's either been meetup or FetLife. I used to have monthly "Plinky" (for Polyamory & Kinky) chats I would host in local parks with an eye to discussion of the intersection of kink and polyamory. Most of the time it was really just a polyamory discussion as kink in general is a lot easier to deal with than just the complexities of interpersonal relationships.
Bumble is great in Europe for this. Many poly people looking for friends... And get in as just friends and this world opens up. Other than that I meet a lot of people through kink. Kink clubs, leather parties, shibari / Kinbaku meetings... Etc. There are some poly meetups... But they tend to be only new people to poly that are often Chaos or need to learn more, also tend to be a bit young for me.