The majority of those with advanced degrees and living in the Western Industrialized countries do not look upon LGBTQ humans as disordered.
This is only partially true. It is no longer classified as a disorder because of lobbying in the DSIM (I think that’s the right acronym, the medical encyclopedia of disorders). What changed was not medical understanding, but rather how something was referred to. This is sort of like if everyone started calling schizophrenia a lifestyle choice, that wouldn’t make it any less of a mental illness.
Priests are generally well educated, many having post grad degrees.
Your implication here is a variation on the “No True Scottsman” fallacy.
Have you ever witnessed a priest refuse to offer the Eucharist to a person that is gay?
No, because that is not the priest’s job. It is the obligation of the individual to assess whether or not they are in a worthy state for the reception of our Lord’s body, blood, soul, and divinity.
I was not specifically speaking of gay marriage, but of supporting loved ones who are in a loving and long term monogamous relationship and happen to be gay, not straight.
The degree of commitment in the relationship has no bearing on the reality that homosexual acts are intrinsically immoral. I’ve never understood why people use this argument. They are immoral because of the nature of the act, not the emotions of the individuals involved.
What I thought when I read the OP is that this couple is lucky to have such a loving family. I would never advocate for the Church to perform marriage ceremonies for gay couples since it is still forbidden, however; I see no harm, much less scandal, in attending the civil ceremony or other religious marriage celebration for those that are in love

and wish to let it be known that they are in a lifetime relationship.
If they were truly in love, they would desire the best for each other, which would preclude them from engaging in an act which is contrary to the most basic biological realities of the human body. I don’t need faith to show that homosexual acts are contrary to human nature, science is more than capable.
As I, and others, have said repeatedly; the scandal come from giving tacit approval to the ceremony by attending. Since he was there, it means that he believes there was something worthwhile in the ceremony which warranted his attendance. There was not, the only thing he attended was a sinful rejection of the nature of God’s creation. The scandal comes from his apparent (or real) approval of the farce. He was essentially saying that their delusion is valid.
Yes, I understand that my stance is not that of many traditionalists, so be it.
This has nothing to do with being a traditionalist, and everything to do with being a Catholic. The Church condemns homosexual act, as well as the concept of “gay marriage.” This has been a constant reality since Christ walked the Earth, and will continue to be one until He comes again. It is not simply some “traditionalist” viewpoint.