To me, the main issue is giving in to the left-wing identity politics Zeitgeist, dividing people into "homosexuals" and "heterosexuals", a distinction that simply did not exist before the late 20th century. Homosexual behavior was always known, but was simply classified as a kind of sexually deviant behavior, not essentially from deviant sexual behavior between men and women (Romans 1:27).
What's really bizarre to me is how the Left has adopted the idea of an "innate" sexual orientation, while rejecting all the (at least equally strong) evidence for other kinds of innate psychological differences, e.g. between men and women. Of course, it makes sense at a certain level, since the real purpose is to undermine traditional Christian morality: if the Church recognizes and accommodates male and female differences, then they argue that these are social constructs; if the Church claims homosexual behavior is deviant and unnatural, they they argue that homosexuality is "innate" and therefore "natural".
From the scientific perspective, there is actually much less evidence for the heritability of sexual orientation than of sex-based cognitive differences, e.g. if an identical twin is homosexual, there is only a 75% chance the other twin is homosexual.