I believe that it is fair to conclude from this that Catholics, as members of their Church, have an obligation to do all that they can to "defend these values, for the good of men and women and for the good of society itself." Unless and until the Church specifically states otherwise, good Catholics may disagree on how to apply the principles the Church outlines in documents like this one, but they must ask themselves whether they are disagreeing for the sake of upholding a right understanding of marriage or for the sake of soothing their conscience so they can avoid personal inconvenience.

You ask why there is a difference between attempts at same-sex marriage and other types of invalid marriages contracted between heterosexuals. I think the answer is that there is nothing that inherently prevents marriage between heterosexuals, indeed marriage was created to be heterosexual, and so invalid marriages between heterosexuals could foreseeably be regularized at some point. So, while there may be a need for family members to avoid a particular heterosexual marriage, there is no need on a social level for a vendor to decline to offer services (unless he feels strongly that he should in a particular situation).

The same does not hold true for homosexual unions. Homosexual unions are invalid by their very nature and can never be regularized. Indeed, by their very nature, they are not merely destructive to the individuals involved but to society as a whole. If they become "normalized," such "normalization" may well destroy civilization, and that is not an exaggeration. For that reason, I believe that all who are asked to participate in any way in such an event should refuse because such an event threatens not only the family but the larger society, of which the family is the essential building block.