Is homosexuality learned or inborn?
Science has not yet established the degree to which homosexual tendencies are learned or inborn. It is likely to be a combination of both factors, like alcoholism is. In the latter case, there is a genetic predisposition to alcoholism, but it takes a process of conditioning and experience with alcohol to develop the addiction.
Sexual drives are built into the human race biologically, but among humans sex is subject to a great deal of cognitive conditioning. Responses to particular stimuli (body shapes, facial features, hair colors, clothing) are largely learned and vary widely from individual to individual, and even from time to time in a single individual's life. This degree of cognitive involvement in sexual behavior is not found among lower life forms, whose sexual behavior is almost entirely instinctual and has few cognitive factors involved in it.
There may be genetic, hormonal, or neurological factors which produce a predisposition toward homosexual desires, but some degree of learning and conditioned response is almost certainly involved (as in human sexuality generally). Because of this ambiguity, the Catechism of the Catholic Church states that homosexuality's "psychological genesis remains largely unexplained" (CCC 2357).