||You are allowed to go to their wedding even though it has mixed seating, because of the Mitzva to be Mesameah a Chatan and Kallah. They should not suffer because of the arrogance of their parents, to not make them a wedding they wanted. I know, many parents are embarrassed from their friends that nebach, their children became "ultra religious" and so they feel they must show their arrogance. It is tough to be Jewish, when you are not completely religious.
But if they themselves wanted mixed seating, then you cannot go. (there are exceptions to this rule - ask your Rabbi directly when it is relevant)