Most poly people who meet others on dating sites put something about ENM/poly in their profiles, so potentials can know before they even swipe. Just imagine how they'd feel if they were mono and fell for you, and then realized they'd be permanently sharing you with others!
I can put myself in their shoes. I'd be pissed. I'd feel you just wasted a bunch of my precious time, and played around with my emotions, my heart.
Now, when I was 19 and home on summer break from college, I was casually dating 3 or 4 guys. This was before we had the term polyamory. One was an old friend turned lover, Tom. One was a brand new guy I'd met at college just as the semester ran down, Mitch. The third was a friend of a friend, Tony, and it was more of an experiment... just a few dates and a few kisses. The third was the brother of a high school friend, Tom 2, and I got drunk at a party at their house and woke up next to him... oops!
(Gosh, I haven't thought about that summer in a while; I was feeling myself lol)
This was mostly all so casual, and we were all so young, I didn't think anyone needed to know about the others. Turns out they were all summer flings except for Tom 1.
But as an actual full grown adult, busy with a job, with ideas of marriage, family, etc. looming, yes, it's definitely only polite to let people know who you actually are and what you're seeking right up front.