Many of us have been there...
Jessie:
As others have said, many of us have been there ourselves or been through numerous times of seeing our mates dealing with their polys (us!) strange ways... There isn't really a single way of working through it, but there are some commonalities I'm happy to try to share with you if you'd like...
I've been poly in brain since I was born, and poly externally for about 20 years now (yes, I was more than a little precocious for my time...). And even being poly myself, I'll admit it's still a little "odd" when my mate(s) actually go out on dates with others. I'm not sure if I can call it jealousy as most describe it (I don't envy their relationship, I just want to see that movie with them, too!), or even fear of loss (Oddly, I don't fear -- maybe this sounds arrogant, but I don't want someone who doesn't see my worth enough to hold onto me...).
But the biggest factor by far that *I* have discovered is (and this should really not surprise you?) is whether the other person is fundamentally worthy of your respect. If my fiance' dates a nice guy who seems to genuinely not wish to come between us, I'm hunky dory -- Yeah, stay over, borrow my tools, whatever... but if he's a total jackass who is clearly only going to hurt the woman I love deeply, I get my hackles up quickly. I don't believe in "bans" or "vetos" per se, but I do expect my mate to listen to my opinion when she's caught up in New Relationship Energy and totally blinded. I've been there myself, and know ALL about NRE making you think the scankwhore is Cinderella...
One of my agreements with my current mate is PLEASE DO tell me when you think I'm dating a scank, because I probably won't see it, and she's likely to show her true colors to my mate (who she's trying to push out) MUCH earlier than myself (who she's trying to impress).
I have a proclivity for dating mono women (not by intent, but because most polys don't seem to agree with my version of respectful, limited poly). Yeah, I know, I'm giving myself a lot of headaches with ending up dating mono women, but I mention it because it does compound all of the above jealousy issues... On top of "human" jealousy, I get to contend with "I don't REALLY understand why you want a second anyway.... AND she's a scank, by the way."
I will try not to speak for my mate(s), but as I said, I can try to give you a list of things that seemed to help based on my perspective.
Take care, and love well...