1. In order to allow gay marriage and have it make any sense, we as a society have to change the purpose of marriage from being an inter-generational commitment made by the couple for the benefit of others into defining what makes an individual personally happy. By definition, once they aren't happy anymore, there's no logical reason to stay married. If we institutionalize the concept that marriage is permanently changed to being about one's happiness rather than an inter-generational commitment to your spouse and both of your families, every marriage will be vulnerable for break-up. We know this because the gays seek marriage equality, which goes both ways. So all marriages will be about personal desires and happiness. Obviously, this has been happening over decades, but this change makes it permanent.
2. Gay marriage promotes the idea that there are different kinds of families and all are equal. This takes away whatever pressure was left to keep marriages intact because, once 2 men can marry, it is impossible to say any situation like divorce or living together, or whatever is bad. It ushers in an era of low and no judgement which loosens the bonds of marriage for everybody since we will have marriage equality.
3. If someone devalues and changes something you have that is sacred you are harmed. For example, if a person could just become a Supreme Court Justice without any intellectual heft, legal training, or political acumen whatsoever, BUT by law must have JUSTICE EQUALITY. The fact that person would be called Justice hurts the ones who are qualified and have the unique talents. No longer would saying "I'm a Justice" mean something special and specific because legally it could mean something totally different. The key here is if there can be no distinction between a person who went to law school, clerked, was a judge, and now a Justice, versus a person who was a basketball coach who is now a Justice. It would change how people see themselves and how others respect them if you forced them to be "equal" with unequals.