Although I'm not someone who has given up a child for adoption, I have helped my ex father in law get in touch with his biological family. Sadly we were about 20 years too late to meet his mother as she died in 1986. Although he said if he found her he never wanted to actually meet her, I know he was pretty devastated to find out she had died before he had really started looking for her.
This week he and his wife have been in Australia visiting various biological relatives and have been having a whale of a time meeting all these random people. The whole lot of them have been overjoyed to have finally found this missing piece of their family. Some knew of him and some didn't, but they have all welcomed him with open arms.
It really is a personal thing. My ex FIL had fantastic adoptive parents who loved him very much. He definitely never wanted to look for his biological mother while they were still around. He knew a basic story about his mother - which had been embellished by his adoptive Mum, probably to make his real Mum sound a bit more fairytale. Tales of a long lost love who lost a limb in the war and broke off an engagement and her following him back to their country of origin. For years he thought she was American or something and had looked down several wrong avenues. I found out she had married just a year after his birth (not to his father though) and they lived a few miles away from him for about 10 years before moving to her home country of Australia.
I think no mother who gives up a child ever stops thinking about the child they gave up, all the what ifs and wondering how they are, what they are doing etc. It would be great if every adoption story had a happy ending, but sadly not all do. I think it's worth remembering that when someone gives a child up for adoption, they are doing it on the assumption that it is for life, that there are no 'returns' and that they are doing what is best for that child. It's great if people want to meet up and have some kind of relationship, but it doesn't always work out, and sometimes too much time has passed to make it a working relationship.
Try not to dwell on it too much, I know it is probably mainly down to the rush of emotions you are feeling being a mother yourself now, like you say, the feeling of not knowing how someone could give their child up and never see them again is a hard one to fathom.
Perhaps in time your husband may change his mind, and by then it may be too late to meet her in person, but there is the adoption contact list where relatives of adopted people can put their details on - like siblings, so that should the adoptee decide to look at their file etc they will see that they have family wanting to make contact with them.
At least you and your husband have discussed it, rather than it being one of those untalked about topics. I'm sure in time you will move on from these feelings, but talking to each other about it is a step in the right direction, for the both of you. It is a tough subject and I'm sure your husband understands how you feel in light of the baby being on the scene, I'm sure he has feelings about it all, having been the child given away - for a better life.
Best of luck to you x