no. 2284 for the year 2003 which includes the following:

Does Islam command men and women to remain separate during marriage contract ceremonies and wedding ceremonies that may be held in mosques? What is the ruling if these ceremonies are held in mosque reception halls with family and friends of the bride and groom mingling in a respectful family atmosphere that is free of any indecency?

Allah Almighty says:

"Say to the believing men that they should lower their gaze and guard their modesty: that will make for greater purity for them: And Allah is well acquainted with all that they do. And say to the believing women that they should lower their gaze and guard their modesty." 
[Qur`an 24:30-31]

Based on the above verse, mixed gatherings are not prohibited but permissible and required especially if the cause of the gathering is noble, if both genders abide by the teachings of Allah in regard to lowering their gaze and if women wear the legal attire which should not be transparent or cling to the body.

The Ruling 
There is no objection to men and women attending a marriage contract ceremony as long as they observe the teachings of the Shari`ah and that these ceremonies be free from temptation or anything that incites lust.

Allah Almighty knows best.