The sexual area of their brain responds to men (more so) than woman. Science is not advanced enough to determine exactly
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The Bible says that homosexuality is unnatural.
"For even their women exchanged the natural use for what is against nature. Likewise also the men, leaving the natural use of the woman, burned in their lust for one another, men with men committing what is shameful, and receiving in themselves the penalty of their error which was due." (Rom. 1:26-27).