I do see where you are coming from about not wanting your new dc to have to meet him but i'm afraid that is not realistic, there will be an occasion where your dd2 has some form of meeting and isn't it better for her to see you relaxed about it rather than ushering her away? DD1 will probably want to say " hey little sister, this is my Dad!" and there's nothing wrong with that, dc's shouldn't be aware of our feelings towards our ex's.
Dh and I have 5 dc's and 3 ex's between us, one of whom is a very unpleasant addict, the other two have has their moments but not quite in the same league.
But they are the dc's parents (however crap they may be) and however much I want to protect the others, when the dc's come home from contact, I'm all smiley and polite on the outside when dc's are there, letting the big ones show the little one's off to their other parent.
I take the dc's home and bleach them afterwards though 
Seriously though, it makes my skin crawl having them say hello to them and ask questions about my dear, dear innocent babies and I hate that they are being exposed to such people even in such a small way, but that's how our life is and everything is an education to us all.
But I have digressed.
One day, marl, there will be weddings, parties, christenings and although it seems a long way off, if you don't deal with it now you will still be struggling with it then.
My step Dad collapsed at my wedding because he couldn't cope with my Dad being there. He'd been married to my Mum for 15 years by then 