I've been known to have more than one freak out with the women my husband has dated/attempted to date. What works for me is to get to know the other woman. Once you see them as a whole person rather than just the outside package somehow it gets easier to accept and even enjoy their relationship.
I've asked of my husband that things procede slowly until I get to know any potential partner. We don't have to be best friends or anything but I do need to know that there is a mutual respect for each other.
I also have worked on my way of looking at things. If my husband is dating others and still wants to be with me it means that he really does want to be with me and isn't just with me because he "has to" be. Strangely enough I'm more secure in our relationship since we have opened it up than I was for years before.
Good luck Kk!
Everything will be ok in the end. If it's not ok it's not the end.