From your update in the comments, I'm assuming you've spoke to him by now.

I'll cut the crap in this comment and just give 2 different opinions:

1) IF they used a sperm bank as a go-between, you're really overreacting in my opinion. It's his body, and if he doesn't want anything to do with the kid? That's completely fine. As a male, the idea of helping someone create life (of course in a non-legally binding way where there is no sex involved) is kinda nice in some weird way. 
It was 100% a bit of a dick move not to tell you, but, if they used a sperm bank and your fiancé has decided not to be involved with the child as if it is his child, then just postpone the marriage, and try to work around this, let him show you if he truly cares. I don't think this is worth fucking up a relationship that otherwise seems so good.

2) IF they fucked... just leave. There's no workaround. 'Insemination' is stand-in for 'we did sexual shit together but Peter didn't want me to fuck up your engagement.' This is really the more likely option, so in my opinion, just leave unless he can get some sort of proof he used a sperm bank.

Obviously those are my opinions, expecting downvotes for that first one, but oh well. Good luck OP.