Is it a right to change the culture?
Here is a very simple argument for the protection of the tradition of marriage. It has nothing to do with anyone's rights. It is very very important to teach young people the value of the traditional family above all else. This is what the preservation the tradition of marriage does. If society does not honor it young adults will not. The advantages of the traditional family are numerous and demonstrated throughout history.
Sometimes the best laid plans don't pan out. Families fail, a mother is widowed, a homosexual relative is left to the care of siblings etc. People in such situations should be admired and respected for their perseverance and responsibility for maintaining but children should never be taught that this kind of parenting is as good as any other. They need to know otherwise so that when they go out in the world and attempt to put a family together they will not be under the illusion that anything works.
The liberal narrative attempts to extract what they believe are the essential elements of a good family; support, caring, teaching etc., and argue that these elements can exist sufficiently in any form. They thumb their nose at the bible and the wisdom of the ages and even current studies on the subject. They believe that they are much smarter than everyone else and will rationalize anything and everything. I had a professor once who remarked one day with a little smile on his lips that there is no one in the world smarter than a grad student. What he meant by that was a grad student is at the very peak of his brain power but lacks the wisdom to know the limitations of his knowledge in the real world. They are naturally somewhat arrogant. In time they find wisdom. The liberal on the other hand suffers from a kind of perpetual adolescence. They may or may not be smart but wisdom seems have passed them by for they certainly have no idea of the limitations of their intellect.