JB,
I sympathise with you, I had the same situation with my little boys birthday a couple of months back where I told my wife I didn't want the new bloke there that she had thrust into their lives so quickly and thought she and he should respect this.
In the end I forced myself to go purely for my son and totally ignored the new bloke as he did me ;-) it was awkward but luckily one of my best friends came and also a couple of the mums from school so I didn't feel too out of place.
If you can take someone then that will probably help your situation
Its still difficult for me at times, I hate going round to the exes place although luckily I see my wife very rarely as most of my exchanges are done at school or with the aupair, as for the new bloke, I have seen him twice since they got together back in August
even though he's bigger than me I will tell him where to go if he ever tries to speak to me in a pally way in the near future.
Make sure you go even if its difficult.
Good luck
Jon