Being gay is definitely a biological thing. While it is debatable whether being gay is a gene or due to hormones the person is exposed to in the womb, there is no real evidence that being gay is due to nurture or upbringing. People are simply born gay and it is not something they are able to truly change.
The vast majority of people who self report as gay have said that they have in some way identified themselves that way for as long as they can remember. In the past, homosexuality was much more taboo, which may have discouraged people who are homosexual from allowing them to process their thoughts and feelings properly.
Life is based on choices. People are given the gift of free will and make choices based on this gift. I believe that homosexuality begins to develop in men and women over a period of time. The attraction to the opposite sex then develops into a way of life. Therefore, it is an internal attraction as opposed to nature.
While homosexual tendencies may be natural, the choice to act on and indulge those tendencies is not. Many behaviors stem from an inherent predisposition towards something, but that does not make them natural, unavoidable or acceptable. A gay person chooses to give in to their sexual urges, which makes the behavior the result of a decision, not the result of nature.