Funkyj, I'm a little late to this thread, but I wanted to make a suggestion.
Please don't judge yourself too harshly on how you feel about this particular potential relationship. Try not to make a universal conclusion about yourself and where you may or may not fit in a poly situation out of just this one experience.
I have different experiences and reactions to the various relationships or dates that my OSO gets into. Some are no big deal. Me and poly? I got this thing handled. *snap* Then he dates someone else and my reaction is to want to get the hell out of this crazy poly situation.
In other words, I can see how your reaction to your husband wanting to date a woman who is specifically looking to find a soul mate with whom to bear children would freak you out - even if the actual situation turns out fine.
On the flip side, your husband might date a woman who is already married with her own children and only looking for a part-time mate, and you might feel no weirdness at all.
I see lots of other good advice in this thread. My 2 cents is to go easy on yourself if you can.