Actually not all members are allowed either. Only those that qualify by getting a "temple recommend" from their Bishop. There is a standard of adherence to gospel principles and commandments that is expected of any member wanting to enter the temple. A "temple recommend" is a small piece of paper about the size of a credit card that has been signed by two Church leaders. When someone enters the temple there is person right inside the entrance that one must show their current temple recommend to. 
 
I can understand the feelings that go with not being able to attend one's daughter/son's wedding. My parents were in the same situation when they were married in the temple. Neither of their parents were able to go with them. It was a sore point at the time, but as time has gone on it has faded in the past - esp since my parents have maintained excellent and normal relationships since that time. The temple wedding was the only time where there was a feeling of separation. After that the church is actually a very big reason for encouraging and strengthening very strong family relationships through the years. I hope your daughter will be able to look past this one time of disappointment and realize that as a result of her son's decision that he will most likely have a stronger bond and relationship with her as a result of his membership in the church than not. 
 
Good luck with the wedding!