Because until recently, no one thought it was an issue. Other than obvious absurdities, like Nero marrying his horse, marriage has been a man to a woman. Occasionally a man to multiple women.
Until recently, most voters would have voted against same sex marriage if asked for their opinion. For example, proposition 8 in California (in the United States) said that same sex unions were not marriages in 2008. This was in response to a court case that said that the California constitution implicitly made same sex marriage legal. Without that, there was no need for a referendum.
Until 2003 and Lawrence v. Texas, it was technically illegal to participate in certain sex acts often associated with gay partners in some places in the US. These struggles may not have had a direct effect on people in Europe, but such things often have indirect effects. People who were satisfied with their lives start wondering what would happen with a referendum in their location. Why worry about a marriage that you wouldn't be able to consummate legally?
Until comparatively recently Hollywood would only show openly gay characters for comedic relief. More modernly, movies like Brokeback Mountain have depicted gay characters in dramatic roles. We've gone from the gay subtext of the X-Men to openly gay actors appearing as gay characters. Society has moved.
Until recently, same sex marriage was primarily investigated in science fiction. There was no need to ban it. No one thought that it was a concept, much less legal. In ancient Jewish law, a marriage could be set aside because there was no issue (children). Obviously, same sex relationships are not going to result in children without outside participation (adoption; surrogacy). So with that view of marriage, same sex marriage wasn't even considered as possible. So why ban it?
Modernly, we've romanticized marriage. It's not something that one's parents would arrange to link your family to another and produce healthy children. Marriage is something that two people who love each other do to express and prove their love. Two people of the same sex can of course love each other romantically (ignoring any arguments as to whether that is natural or perverse or whatever). So from that perspective, same sex marriage is quite possible.
It is only with that possibility that it becomes an issue that could appear in a referendum. People thought they knew what marriage was. It was only modernly that it seemed it might be something else. So now people find themselves arguing over the definition.