He is saying that he believes the Church knows its wrong.
He clearly is not so sure in his own circumstances.
But even this needs to be deconstructed.
The Church infallibly teaches that homosexual acts are, objectively speaking, wrong.
That’s quite a long bridge away from saying attending the civil union of a gay couple is always and everywhere gravely immoral if knowingly done.
Even to go would be to celebrate it, since it’s designed to be a celebratory occasion.
Funerals are also a public ceremony. My last one wasn’t a "celebration."
The use of words in this way sounds like a bit of an autisticism to me.
Are plane “black boxes” therefore black?
I even in some way wanna say “congratulations”.
Yes, the establishment of a committed faithful caring relationship like being a godparent, an adopted parent, becoming a “blood brother” in battle - these are indeed honourable values that make us glad despite the fact that we may be sinners in other things at the same time.
If you are close to the guy and he has honoured you with an invite then don’t blow your chance of being able to influence him for the good by that ongoing relationship of respect.
You are in a different situation from those whose absence would have no impact on their relationship with the person involved. Then again, if your friend does not know your position then maybe you are not that close anyways? In which case your absence would not be that significant if you provided a reasonable excuse.
If he is close to you and knows your position already then, if I was him, would respect you more as a Catholic for coming. Jesus did regularly “celebrate” with thieves, gluttons, and prostitutes didn’t he.
You would then be more like Him than a number of Christians I know