Except that homosexuality has been documented in nearly every single animal species on this planet.
Yes, but the existence homosexuality is
incidental. Sexuality evolved to be reproductive, hence the
final cause of sexuality is reproduction. The fact that sex feels good is a mechanism of sex, but a mechanism isn't the same things as a purpose, which if the purpose of sex in nature is to reproduce, then homosexual sex cannot be natural in its final cause.
The final cause of sexuality
in nature as put there by God, is procreation. To frustrate this ordered purpose is sinful because it's a wilful distorting of God's purposes expressed by the movement of things within nature. This is the same reasoning that motivates the prohibition on contraception, because contraception by its explicit purpose frustrates the intended reason (or natural goal) for sexuality.
Natural law is not about what exists in nature, but the movement of things within it towards their intended definite ends. The idea is that one can derive objective moral principles by the observation of such ends. The fact that distortions of the final cause of things in nature exists, is not an insight that centuries of scholastic thought mysteriously overlooked.