The real reason...

...separation of church and state is important: each has its specific function within society, and when one crosses into the domain of the other, it's a recipe for catastrophe...

The country's parliament voted through the new law on same-sex marriage by a large majority, making it mandatory for all churches to conduct gay marriages. Denmark's church minister, Manu Sareen, called the vote "historic". "I think it's very important to give all members of the church the possibility to get married. Today, it's only heterosexual couples."

Denmark has been a pioneer in gay rights since 1989, when it became the first country in the world to offer civil unions for gay couples. 

You see, 'tolerance' was never about live and let live.  It was about creating the environment to force affirmation of that which was previously shunned.  Just as secularists rail against examples of "blue laws" forbidding, for example, alcohol sales on Sunday, the faithful can rightfully protest the State has no business forcing a church to conduct a mockery of a key ceremony.  The more militant secularists may reply "well, that's just the shoe pinching the other foot."  That may be so, but this particular payback involves spitting in God's face, which is dangerous for any society that hopes to continue...

(HT: Vox)