If YOU feel very strongly about your relative's sojourn into the committed form of this behavior pattern & "alternative lifestyle", perhaps a clear but dignified conveyance of your convictions before the fact would be appropriate??
Then he would understand if you do not attend the event.
This is not a "wedding" or a "marriage" - I would not take a guilt trip over either choice that you make.
In the final analysis--
If your presence at the event, and your gifts or good wishes would violate your spiritual convictions and moral conscience by condoning his lifestyle choice---
Be faithful to your own conscience & principles-- politely...
Regards
I definitely agree with that.
Whichever choice is made, make it, stick by it, don't waffle on it and don't apologize for it.
While you are responsible for a certain amount of consideration of their feelings, they are responsible for a certain amount of consideration for yours.
I'd also say that speaking to other members of your family about how they plan to address the situation might be helpful.