>>6 Couldn´t you also ask "Why should he"?
Anyways, to answer your question - It´s stated in the Bible.
"And you, be ye fruitful, and multiply; Bring forth abundantly in the earth, and multiply therein." (Genesis 9:7) - this doesn´t work if someone becomes gay.
Also "Do not have sexual relations with a man as one does with a woman; that is detestable" (Leviticus 18:22)
--> So humans being gay is obviously not in the sense of God.
Of course this solely is no proof at all. Just read the entire Bible and you´ll find more about this, Dear Kyouka.