Am I supposed to just be a good sport for my SO, be all normal and nice and just coast through it?
Yes! Please be that girl!
Look - there's one of two things here. Either everything is as he says it is - just a friendship - in which case you should go and be nice (I don't know how you can dislike her if you haven't even met her) and be the lucky girl who has the guy on YOUR arm.
If it's really an inappropriate friendship then this party should give you an opportunity to find that out, and wouldn't you rather know sooner than later?
I'll tell you my experience - divorced after a long marriage and ex's MLC. Currently dating a guy (long story - 3 months after we met he was diagnosed with stage 4 lung cancer, living with me 2 years later, not terminal yet but will become so). Not sure if we would be together if he were not so sick, although there are lots of good things about the relationship. But one of the bad things - and a hard one for me - is he is SUPER JEALOUS. I have never had a jealous boyfriend before and I find it really annoying. I'm in my 60's and was dating for 10 years before I met my ex-husband and for almost ten years before I met current boyfriend, so yes, I have ex-boyfriends and lovers. And almost all of them - except my ex husband - are friends of mine now. Not in any inappropriate way, just friends. Most of them I don't really see often (or at all) IRL but we do interact on FB a bit or email. Nobody's carrying any torches, nothing inappropriate, but if I liked them enough to sleep with them, odds are I still like them enough to share a political meme or a new song or sometimes even an off-color joke..
Last year my mother died. At her funeral the priest who presided was an old high school friend - part of a larger friend group from high school that still gets together once a year. He and I dated (chastely) for about one semester in high school. After high school he went into the Catholic priesthood. We aren't FB friends or anything, I just see him maybe once every few years at a friends get-together. Boyfriend was mad that he gave me a hug after the service was over (the priest knew my mom )!
Then at the reception/wake afterwards in a restaurant, my old college boyfriend from freshman year, who had attended the funeral, sat next to us for part of the time. He and I have remained friends for 45 years but probably have only seen each other in person maybe once every ten years. We are friends on FB but seldom message each other, occasionally comment on each other's posts. He lived with my family one summer and was close to my mom. Boyfriend was mad that he sat with us.
Boyfriend is a little batsh!t crazy and thinks that these guys are looking at him thinking "haha, I had her first!". It's weird and disrespectful and at our age kinda nuts.
Now, it just happened that the other day I was digitizing some old home videos and one was a 2 nd birthday party for my oldest. My husband (now ex) is there, and midway through the old college boyfriend shows up, present in hand. I watched my ex's reaction - friendly, appropriate, nothing weird there. He didn't have a problem with this guy being my friend even though at that time it was MUCH more recent (only 12 years since we had been together rather than 45!). That's how it should be - my husband knew that we were only friends and he wasn't threatened by it.
It sounds like your boyfriend has nothing to feel guilty about and you need to get over these jealous feelings for the sake of your relationship. And if I'm wrong, and she's really someone who is a threat to your relationship or that your BF is carrying a torch for, you'll find out if you go to the party. Just don't embarrass him like my boyfriend embarrassed me at my mom's funeral. And don't drag old business from your marriage and your ex-husband's infidelity into your new relationship. Ok?
(Now, there IS the fact that it's a baby shower and that's just about the worst kind of party anyone can be invited to, and I'm sorry if you have to go and play those stupid games but just go and give her a chance. You might find out you feel a lot better about things when you meet her.)