Seeing your child married is a special moment. The presence or absence of birth parents can take an already emotionally charged day and push it over the edge. The best way to handle things is to remind yourself that it is your child's day and you are there to help his dreams come true.
If birth parents are part of your child's life, your child may want them involved in the wedding. There is the potential for a lot of hurt feelings, however. Think about how you would feel if your child asked his birth mother to accompany you as you walk him down the aisle? How would you feel if the birth parents or extended birth family were in family photos?
Your daughter- or son-in-law is a lot like an adopted child. He came from elsewhere and has now become an integral part of your family. If you are able to welcome him as a child of your heart, just as you did with your adopted child, you are sure to have a wonderful, loving relationship with each other.
There are no set rules for how to handle the situation. Your child needs to do what feels right to him; your job is to support him, smile and remember it is his day, not yours. If you feel very strongly about some things, it is okay to express your feelings, but remember you can't make the decisions.
If your child is not in contact with or has never been able to find his birth parents, his wedding can be a bittersweet day. He may be sad they are not there, and he may be angry that he cannot share this moment with them. Your child is an adult, and must cope with this situation in whatever way is best for him. You can offer support, a shoulder to cry on, and your undying love.