You don't have to open up your marriage, period, and you can also choose to give yourself more time... if thinking of your husband being in love with another woman is too much for you right now, you probably shouldn't open up just yet, in fact, as there's every possibility that'll happen. There's also the fact that he broke your trust and it takes a little time to recover from that. If he wants this to work long term in the least messy way possible, it might behoove him to step back and give you more time to heal, learn and process, no matter how much he likes this woman.
There are some great essays on jealousy at this site -- http://www.xeromag.com/fvpoly.html
-- check 'em out, do some reading here, don't let yourself feel rushed!! I worry in part because you don't sound super excited about the foursome... you say you find her "fairly" attractive... and being in a sexual situation with other people, where there's pressure to participate and you're not necessarily totally into it, plus still dealing with fear and jealousy issues and then facing the reality of maybe seeing that loving, so-into-you look in your husband's eyes when he's with this woman and you guys are all together? I could be wrong, but it sounds like a powderkeg.
So, again, my one major piece of advice -- take is slow! Best of luck.