From NARTH's webpage boilerplate...

"It is NARTH's aim to provide a different perspective. Particularly, we want to clarify that homosexuality is not "inborn,""

The NARTH assertion that homosexuality is not inborn is no more valid that a biopsychologist's claim that homosexuality is rudimentarily connected to neurochemistry and neuroanatomy.

Both are assertions warranting empirical investigation, and neither has been unequivocally supported.