If this is true, why should they modify their behavior?
Because we all have a fallen nature. I am not sympathetic to homosexuals. I don't give a damn whether they were born that way. Because I was born a human, with red blood, a descendant of Adam, infected with sin and a fallen nature. And whether I like it or not, I am damned for eternity! The only reason I can live forever is because God came to Earth as a man, and died for me. If homosexuals don't like, well tough. Because that is the message of the Scripture. Unless one repents, all will perish. No excuses are excepted.
Luk 13:2 And Jesus answering said unto them, Suppose ye that these Galilaeans were sinners above all the Galilaeans, because they suffered such things?
Luk 13:3
I tell you, Nay: but, except ye repent, ye shall all likewise perish.
Luk 13:4 Or those eighteen, upon whom the tower in Siloam fell, and slew them, think ye that they were sinners above all men that dwelt in Jerusalem?
Luk 13:5
I tell you, Nay: but, except ye repent, ye shall all likewise perish.
Rom 1:20 For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead;
so that they are without excuse:
Rom 1:21 Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.
Rom 1:22 Professing themselves to be wise, they became fools,
Rom 1:23 And changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things.
Rom 1:24 Wherefore God also gave them up to uncleanness through the lusts of their own hearts, to dishonour their own bodies between themselves:
Rom 1:25 Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen.
Rom 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:
Rom 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.
Rom 1:28 And even as they did not like to retain God in their knowledge, God gave them over to a reprobate mind, to do those things which are not convenient;
Rom 1:29 Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers,
Rom 1:30 Backbiters, haters of God, despiteful, proud, boasters, inventors of evil things, disobedient to parents,
Rom 1:31 Without understanding, covenantbreakers, without natural affection, implacable, unmerciful:
Rom 1:32
Who knowing the judgment of God, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them.