Because God's law said so.
Romans 3:20 KJV
(20) Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.
Here's God's law:
Leviticus 18:22 KJV
(22) Thou shalt not lie with mankind, as with womankind: it is abomination.
Leviticus 20:13 KJV
(13) If a man also lie with mankind, as he lieth with a woman, both of them have committed an abomination: they shall surely be put to death; their blood shall be upon them.
Who's Blood is the issue NOW, though?
Romans 3:21 But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets;
2 Corinthians 5:19 To wit, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them; and hath committed unto us the word of reconciliation. 5:20 Now then we are ambassadors for Christ, as though God did beseech you by us: we pray you in Christ's stead, be ye reconciled to God. 5:21 For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him.
Romans 12:19 Dearly beloved, avenge not yourselves, but rather give place unto wrath: for it is written, Vengeance is mine; I will repay, saith the Lord.
Romans 2:16 In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.
Romans 16:20 And the God of peace shall bruise Satan under your feet shortly. The grace of our Lord Jesus Christ be with you. Amen.
Admittedly... I would really have to focus on all that were I to encounter a pedophile in the middle of his heinous act against a child.
How anyone can allow themselves to act on such an impulse - to so easily destroy the preciousness of our greatest joy - our children...
Admittedly, my first impulse in the face of that is that other joy - the joy of swinging a baseball bat with everything in me and then some...
Thing is; one does not find the Apostle Paul going on about much of these issues.
And he ministered far and wide within the mighty Roman Empire.
An empire in which what we are only seeing a very miniscule glimpse of had become the accepted norm at all levels of society.
Human trafficking; rampant, out in the open pedophilia; rampant hetero and homosexual prostitution and or households; murder for sport; you name it; it was all the accepted norm.
And yet, very little on any of that by the Apostle of the Gentiles.
The above passages, and then some; is why.
Where was God's voice in Lot at Sodom and Gomorrah?
Bad enough the guy could have stood even living in such a foul place.
Where was God's voice?
I suspect it was right there in what the Apostle Paul would reiterate centuries later in Romans 1 and 2.
Oh but that baseball bat o how it calls out every fiber of my being...
Admittedly...
As for homosexuals; well that is on them; they are the ones consenting with one another...
Romans 1:26 For this cause God gave them up unto vile affections: for even their women did change the natural use into that which is against nature: 1:27 And likewise also the men, leaving the natural use of the woman, burned in their lust one toward another; men with men working that which is unseemly, and receiving in themselves that recompence of their error which was meet.
Until the wrath to come, resulting in...
Hebrews 9:27 And as it is appointed unto men once to die, but after this the judgment:
Revelation 20:15 And whosoever was not found written in the book of life was cast into the lake of fire.
lain: If Clete want's to qualify his remarks he's free to do it. 
