Hi Michael,
What do you mean ‘the why of morality’?  Are you referring to the social morality or the Catholic morality?  
“If God has an agenda in sexual ethics for some kind of divine holiness,..”  God is not a finite man, “I AM WHO I AM.”   I wonder if God has an agenda in anything?  
I am not a homosexual, thus I am not able to talk about it one way or another.  However, we, human need to be/live, i.e., ‘to be is to love’.