Word meanings change over time as well. Just because a word BEGAN as meaning one thing does not stop it from growing to change into a new meaning. If you know about word roots and etymology then you should be comfortable and knowledgable with that idea as well.
Personally, you're right about people putting up a fuss about creating a new name for "gay marriage". I can see why some would think it is not a big deal (and logically it isn't a big deal). But there are certain connotations with marriage, and having to make a new word for "gay marriage" really just emotionally makes some people feel that "gay marraige" doesn't live up to "straight marriage", like it's a second-rate type of marriage. Whether this idea is right or wrong really depends on personal preference, but it is a matter of fact, and one cannot ignore basic instinct to believe that.
Personally, you're right about people putting up a fuss about creating a new name for "gay marriage". I can see why some would think it is not a big deal (and logically it isn't a big deal). But there are certain connotations with marriage, and having to make a new word for "gay marriage" really just emotionally makes some people feel that "gay marraige" doesn't live up to "straight marriage", like it's a second-rate type of marriage. Whether this idea is right or wrong really depends on personal preference, but it is a matter of fact, and one cannot ignore basic instinct to believe that.