Shall we assume that this part of the “gay” wedding vow exchange is omitted – "Should there be anyone who has cause why this couple should not be united in marriage, they must speak now or forever hold their peace
."?
Regardless, that maybe just what your friend needs from you if he is planning his “gay” wedding ceremony.

RE your last point, while I don’t know if I know a gay person, if I had such a friend, you may rest assured there would be no doubt long before the “ceremony” got close, the friend would know where I stand.
Regarding your first point, if I’m correct, this “forever hold your peace” is not included in a Catholic wedding as attendees are not participants and have no authority on the administering of the sacrament. If I’m incorrect, this “phrase” is essentially customary and not taken seriously. If it were, we all know of friends and relatives (especially fathers of the bride) that would have felt cause to express their opposition. But even if they did, their opinion would be of no weight.
But you have a good point. I think if someone were to attend either a gay wedding or an illicit wedding (i.e. previously married Catholic) it probably means we should prior to the wedding make sure we have personally expressed our views to our friend or relative lest the person getting married were to infer something beyond intent. I don’t know if I’ve done so in the past but your comment will cause me to do so in the future.
Right or wrong, I live my life with little regard to what people “think” about me. I’m more concerned about what people “know” about me. This is why I wouldn’t not attend because other attendees might “think” I’m pro gay wedding and would be inclined to attend to show affection for my friend or relative. At the same time, if I choose not to attend (where I think I am in this hypothetical situation), I’d do so because of my own personal views on the subject.