It is a fact that the Catholic Church condemns same-sex 'marriage' on account of the sexual activity which takes place within such a union. No one can dispute that this is the teaching of the Church on this issue. 
The Church's mission is to convert all souls to Christ and part of that mission is to oppose all constitutions, legislations, court judgements, etc which conflict with its teaching. 
In this day and age, such opposition requires finance. The KoC being an institution within the Church, whose members support its doctrine is quite entitled and free under the Constitution to provide financial assistance to the Church in combatting what it determines to be a universal moral evil. Remember the Church is concerned with much more than the physical well-being of mankind.
There are many secular bodies who provide financial support for those who oppose the Church's position (e.g George Soros) I don't see you calling them or criticising them.