This is what happens when politics is controlled by religious zealots. I am Christian myself but I believe that there should be separation of church and state.
The DUP are basically idiots. Even Sinn Fein can rightly portray them as extremists on this issue. For SF, the underlying strategy is to box the DUP in at the loony end of the social policy spectrum so that moderate Protestants won't vote for them, thus making SF the biggest party in NI and preparing the way for a UI vote post-Brexit.