I've been in poly relationships on and off for 14 years. I'm not a bull, but more of a cuck, though I used to be the 3rd guy for swinger couples before polyamory.
I like an open relationship where some feelings can be had and admitted to, but I'm not fond of the type of polyamory where people plan to keep things super egalitarian, if the original couple lives together. From what I've seen, that's asking for failure in the long haul, unless the couple has super duper poly and communication skills, etc.....
I'm of the opinion that feeling a crush and bonding is normal for most people that have repeated sex with someone they find attractive, but if the original couple relationship does not take priority, it is less likely to last truly long term.