Will Rogers famously remarked that he never met a man he didn't like. However, I'm pretty sure Will Rogers wasn't polyamorous. So what happens when you're happily polyamorous, your partner—with your blessing—starts dating someone new, and you realize that you do not like this person?
It's a tricky situation. People committed to the ideals of open romantic relationships may feel they're betraying their principles by disliking their partner's other partners. (Let's call them POPs for short.) Polyamory certainly goes more smoothly when everyone gets along. But two perfectly nice people, both loved by a third person, can sometimes be just too different (or, just as often, too much alike) to be close friends.
Once established, strong dislike of a POP isn't a problem easily solved. My only advice is the obvious: Tell your partner how you feel, but recognize these are your feelings and your responsibility. Be polite to your POP in social situations, but minimize time spent in close company. (Your partner should give full cooperation in this.) And no bad-mouthing to your mutual love—confine any snarky comments to understanding pals.
I do have a strategy around preventing negative feelings from happening in the first place: not too much, not too soon. Yes, it's nice to meet the people your partner is dating. But some polyamory literature strongly suggests spending lots of quality time with your POPs, on the theory that it makes them less emotionally threatening. Phrases like "poly family"—a Charles Manson–ish term I detest—are tossed around, implying everyone should want to all hang out together. One gets the impression that poly people are damn-near obligated to welcome new POPs by love-bombing them like a Moonies recruit. I disagree.
No matter how poly-experienced you are, there's a period of adjustment when your partner gets a new partner. It's normal to wonder: How is this going to affect me, my relationship with my partner, and my life overall? You can learn to handle this phase more smoothly and quickly, but you never eliminate it completely. I prefer to pass through that phase first. Once I'm used to the fact of the new POP, then I get to know him/her just as I would anyone else: slowly, organically, without any assumptions of intimacy. I think feeling that one must immediately like the new person as much as one's partner does is what breeds resentment and then dislike. If I never have anything but a pleasant casual acquaintance with my POPs, that's just fine.
Sometimes, no matter what anyone does, you simply won't see the appeal of your POP. It's tempting to explain to your sweetie how this Imperfect Someone is unworthy of him/her. But here's my prediction: One day you will be madly infatuated with someone your partner thinks is a charm-free loser. So take a less-famous bit of advice from Will Rogers: Never miss a good chance to shut up.