A homosexual union is not comparable to a Protestant wedding or any other non-Catholic wedding. The boundaries of lawful attendance at such things pertain to the often religious nature of such events, and Catholics who are invited to attend must ensure that their attendance, which is permitted even under the old canon law for serious reasons (family ruptures being one of them), be passive. This is to avoid given scandal of indifference. The actual union itself is unobjectionable from a moral perspective, supposing that the parties are free to marry. What's more, in the case of baptized protestants, it's an actual sacrament.
A homosexual union, as virtually everyone has pointed out, is not only not a real marriage (and in that one particular regard is more analogous to a wedding between two people who have living spouses) but the event itself is an intrinsically grave moral and social evil. It would be analogous to attending someone's abortion or suicide (both of which are sometimes "celebrated" nowadays). The notion of even passive attendance goes out the window because there is nothing at all in the scenario which can be morally supported.