Sometimes we box ourselves in and cut ourselves off from joy and happiness with preconceived ideas of what something will be like.
In your case, it sounds like you both believe that a daughter will provide an experience that a son can not.
Do not box yourselves in. Open your minds and realize that the experience of parenting any child, whether the child is a boy or girl will be equally fulfilling, exciting, and fun. You will not miss out any experiences with one or the other. In the end, parenting isn't about matchbox and baseball games versus tea parties and all things girl. You will quickly find, the most fulfilling and most important parenting experiences are the ones that have nothing to do with gender at all, but everything to do with the building of the character, values, and education of that child.
Besides, as a mom who has lost a young daughter, and has only boys, I can assure you, something hormonal happens that causes a mother to fall in love with baby immediately upon birth. Short of her experiencing something challenging like post-partum depression, she will not feel this way once she sees them, holds them, smells them and nurses them. Most mothers instantly feel fulfilled and whole after they deliver their babies. It is an incredibly powerful and life changing event with a whole host of unexplainable emotions and physical changes that make it so no other children in the world but hers matter. It will be the same for you too. Especially if you get to be in the delivery room to witness the birth.
For now, clear your mind of these thoughts the best you can and get excited for those babies by getting the room ready, getting cute things that any child/baby would enjoy, and most important spend tons of quality quiet time with your wife.
When the babies come, this phase of your life together as a couple will end, and you will both be entering into a new dynamic centered on the children until they are teens. So many new parents don't understand this (how could they unless they have a wise, experienced person tell them) and miss out on the short time left to have fun without the interuption that comes with the things of parenting. Good things...but very time consuming things that make quiet intimate conversations the exception, and peaceful baths, and quiet romantic dinners a rarity. For goodness sake take the woman on several romantic dinner dates! Escape to a bed and breakfast! Go to a concert! Be spontaneous and have fun because you don't have to worry about finding a babysitter or getting back home by a certain time yet. LOL! Enjoy having a clean house, small laundry loads, and being able to hear the TV and actually being able to watch something other than kid shows.
Whatever you do, do not worry about things you can not change, nor would you want to change. These are afterall the babies you were MEANT to parent -- two boys who will be your own to love, cherish, and teach. What a blessing and what an adventure you and your wife will have. You'll love each and every moment and will probably never dwell on the what ifs of not having a daughter once you hold them in your arms.
Last but not least, if after the boys come, you both still feel that your family is incomplete, why not adopt? There are plenty of little girls out there who would love to have two big brothers and two loving parents.
If her sadness (which is normal) is such that you are worried for her mental health and safety, I would recommend counseling. Hormones may also be at play here. Talk with her OB if you feel she is in a very unsafe place emotionally. She may need some extra TLC before and after delivery. Just keep an eye on her.