However, like being gay or trans, one doesn't always know since birth. It's a process of discovery.
As far as the "danger" of developing feelings... you're describing swinging, or open relationships for casual encounters. Polyamory = many loves--developing feelings for another is quite the point.
However, like monogamous marriages, polyamorous couples/groups set their own rules, and if both partners agree that the relationship needs to be cut off if feelings develop, that's ok and valid, too... as long as they are upfront with -all- parties, including the third who has to get dumped because one of the partners fell in love with them.
Polyamory is complicated and should not be practiced by everyone.