<"Leaving aside the whole debate about whether homosexual acts are intrinsically sinful in a way that heterosexual acts are not .... ">

Well, no, let's not leave this central issue to one side! 

A person who dies unrepentant of willfully committed grave sin, with full knowledge and assent, is, according to Church teaching, facing God's judgement and eternal damnation. Their lives have been lived outside of God's grace and their eternity will be the same.  

To offer a Catholic funeral Mass for a person known to be publicly embracing a gravely sinful homosexual lifestyle cannot but fail to give a false message to others that God, in His mercy and compassion, judges nobody and that homosexual sex acts are acceptable.