The simple fact is that the purpose of marriage (from a secular society’s point of view) is to try to provide stable upbringing for the next generation of citizens.
Gays can’t produce the next generation of citizens, not without assistance from surrogates and doctors, or by adopting children, and so most of them will always be childless. Because of this, they have no need to be in stable relationships, because children are not an unavoidable result of gay couplings.
Now, just because they might be able to obtain children, gays will argue they should be able to get married in order to provide a stable home for the child. However, if they really cared about the children, they wouldn’t try so hard to obtain them when they knew they couldn’t provide that stable home in the first place, so we can see their stated motivations are a false veneer.
Marriage is a religious ceremony and commitment before God.
The civil society may want to encourage marriage for a variety of reasons. The fact that civil society approves or disapproves is not relevant