From what Ive found when looking into the matter is that it can be both a choice and innate, but far more often it is innate.
"There is no consensus among scientists about the exact reasons that an individual develops a heterosexual, bisexual, gay, or lesbian orientation.[75] The main reasons cited include genetic and environmental factors, likely in combination.[76][77] Other factors that may play a role include prenatal hormone exposure, where hormones play a role in determining sexual orientation as they do with sex differentiation;[78][79] and prenatal stress on the mother.[80][81][82]"
"The American Psychological Association, American Psychiatric Association, and National Association of Social Workers stated in an amicus brief presented to the Supreme Court of the State of California: "Sexual orientation has proved to be generally impervious to interventions intended to change it, which are sometimes referred to as ?reparative therapy.? No scientifically adequate research has shown that such interventions are effective or safe."
-Also, gays and lesbians should be able to have civil unions for the simple reason that the only arguments that people are able to make against them having civil unions are solely religious based, and the US is a secular system of government.