The Church does truly believe that Homosexuality is sin because they rather chose to believe in their prejudices and society rather than Jesus, for which their bible was to written to reveal.
On sexuality, and marriage, consider this:-
(Mat 19:11 NKJV) But He said to them, “All cannot accept this saying, but only those to whom it has been given:
(Mat 19:12 NKJV) “For there are eunuchs who were born thus from their mother’s womb, and there are eunuchs who were made eunuchs by men, and there are eunuchs who have made themselves eunuchs for the kingdom of heaven’s sake. He who is able to accept it, let him accept it.”