Because they're human. Sperm-production and menstruation have nothing to do with sexuality -- ask any five year-old who has a crush on his teacher, or post-menopausal woman who's still sexually-active. Why are some people born with a sixth toe or a third nipple; why do men have nipples at all?
if a gay man has a child with a lesbian woman would their child be guaranteed to be gay?
There are so many ways to answer that question. If sexuality were controlled solely by a single gene, then yeah, probably -- however, such a scenario doesn't leave much room for bisexuals.
The fact that sexuality isn't binary doesn't rule out a genetic component. It's possible that sexuality is entirely genetic, controlled by a variety of genes to the exclusion of environmental factors, much like eye or hair color, or influenced by environmental factors, much like height or weight.
And if the gay man can have intercourse with a woman, does that mean he is really bisexual?
If you're talking about seeing the act through to the end, I'm sure you know that the only requirement is warm friction -- getting that from the inside of a rabbit-skin glove wouldn't make one a beastialist with necrophilic tendencies.
If you're talking about getting sexually excited with someone of the wrong sex, is a man really bisexual if he gets an erection during a physical-exam by a male doctor? Is a male really bisexual if he's anything other than completely flacid in the gym lockerroom?
If you are going to make the argument that some people are born to only have sex with the same sex then it stands to reason that they would have sex organs that "go together" as well as no need for the reproductive attributes and cycles of heterosexuals. Why does a lesbian have eggs? If she is "born that way" then the process should make physical and biological sense beyond just desire.
All sex is not reproductive, but all reproduction is sexual. Therefore, any "born that way" gay or lesbian has no need for the reproductive aspects of sexual intercourse. Why do they have them anyway. It's a valid question.
If you're talking about getting sexually excited with someone of the wrong sex, is a man really bisexual if he gets an erection during a physical-exam by a male doctor?
The argument that sexual responses are only attributed to "orientation" is the gay argument, not mine. Thanks for making my point. We are born with a sex drive and certain physical responses to stimulation of genitals. How we do it is a choice -- and emotional as well as mental health play a part in that.