If a wedding will be invalid…I do not go…nor do I give gifts…even later to the “married couple”…nothing to show any “approval” of what in reality is a fiction…(even if they are perhaps in “good conscience” subjectively)…So I can not recommend anyone going to such…
Certainly acting as a bridesmaid (not me of course I am not a woman) would be in my mind out of the question…one then is very clearly seen as approving etc. It is a very public endorsement. And it is also important to note that the maid of honor is actually the official witness legally (etc) (not that the OP was asked to be either…I just note it)
I will love them and will be friends etc…but I must only act truth. Charity yes…but Charity without truth is not charity…
Catechism:
1868 Sin is a personal act. Moreover, we have a responsibility for the sins committed by others when we cooperate in them:
—by participating directly and voluntarily in them;
—by ordering, advising, praising, or approving them;
—by not disclosing or not hindering them when we have an obligation to do so;
—by protecting evil-doers
(Perhaps there are times when attendance can be done avoiding such approval etc…I have seen the argument made in very good circles …[
cuf.org/faithfacts/details_view.asp?ffID=137…(note](
http://www.cuf.org/faithfacts/details_view.asp?ffID=137…(note) there the article goes into many things…including what I noted) but I have yet to judge I should attend)