People are starting to accept that there are gay people and they are not going away. No matter whether they think being gay is a sin or a choice, they are strarting to accept that these people have rights, and that they deserve to be treated like the human beings they are.
It's not hurting anyone, and having state legitimized legal marriage is good for society. It encourages people to spend money (buy homes, settle in areas, pay taxes) and I think has a stabilizing effect on society. Legal marriage helps spouses share in insurance benefits, health care decisions and estate planning. Hopefully it will be the norm.
Same sex marriage is becoming popular and soon it will be the norm across the world. This is evidenced as Taiwan sets to be the first Asian country to legalize the same sex marriage. Though lots of leaders are against this, most of the citizens have been seen practicing this.
There are some countries that do not allow women to drive a car or get an education. These countries are never going to go so far so as to allow same sex marriage. It will not happen in our life time. These societies have so many advances in rights to make that it is going to take time to ever get to same sex marriage.