Mind you, Sorba seems to be arguing against a strawman anyway. According to the APA page on sexual orientation:
What Causes a Person To Have a Particular Sexual Orientation?
There are numerous theories about the origins of a person’s sexual orientation; most scientists today agree that sexual orientation is most likely the result of a complex interaction of environmental, cognitive and biological factors. In most people, sexual orientation is shaped at an early age. There is also considerable recent evidence to suggest that biology, including genetic or inborn hormonal factors, play a significant role in a person’s sexuality. In summary, it is important to recognize that there are probably many reasons for a person’s sexual orientation and the reasons may be different for different people.
So (a) it’s complicated, and (b) it varies. Clearly, talk of a ‘gay gene’ is over-simplistic, but then that’s a term from journalism rather than science (See “Jurassic Park and the Gay Gene: genetics through the distorting lens of the media.“ by A.S. Wilkins in the Journal of the Federation of American Societies for Experimental Biology, vol. 7 no. 13 (1997)).
Rather more important is the APA’s next answer:
Is Sexual Orientation a Choice?
No, human beings can not choose to be either gay or straight. Sexual orientation emerges for most people in early adolescence without any prior sexual experience. Although we can choose whether to act on our feelings, psychologists do not consider sexual orientation to be a conscious choice that can be voluntarily changed.
So surely discriminating on the basis of sexual orientation would be immoral, because it is something over which the individual had no control? If not, in what way is it different to saying sure someone didn’t choose to be black/female/jewish, but we’re not going to give them the job because that’s still what they are?