Or to study biology at all?
There are other animals in which homosexuality occurs. Lots of animals display homosexual behavior. A few also display bisexual behavior.
Some people are born hermaphrodites or with extra or missing sex chromosomes. That is, XO or XXY etc.
One of the biggest impacts on female/male characteristics and the separate but sometimes related issue of sexual orientation is hormones. Turns out, the range of estrogen to testosterone levels is quite varied and looks like a double bell curve. A minority of folks are born with natural levels that are highly sexed either as male or female. A minority are born with natural levels that are about equal- that is that they are naturally androgynous. Most are born with slightly more estrogen than progesterone and are the average female, or vice versa and are the average male.
Also, gender and sexual orientation can be shaped quite a bit by culture. Human sexuality is quite plastic and varies tremendously by culture. There are indeed cultures where the norm is homosexuality, and they practice heterosexual sex only with the intent of procreation, and men and women live separately. There are cultures where the norm is heterosexuality but men and women still live separately and not in the nuclear family grouping with which we all are familiar. There are cultures where the male warriors are expected to be homosexual and that is a method of bonding them as a community of soldiers. There are cultures that have more than two genders, including transsexuals and additional gender categories and these people often have a special place or occupation.
Even under a system that has heterosexuality as the only acceptable situation, about 10% of the population generally is homosexual.
Whether you choose to think that it is acceptable or not is another issue, but it is inaccurate to say that homosexuality and bisexuality do not occur elsewhere in nature, or that they are not considered "normal" in any culture.
God may have decreed that homosexuality was wrong for humans, but it is clear that it is not a purely human trait. Why does it continue when it doesn't produce offspring? Well, in some animals it seems to lower aggression (such as Bonobos), establish a hierarchy, and/or produce bonding, all of which can lead to furthering the survival of a social species.
Personally, I just don't get the problem with it. And even if it is a sin in some religions, the amount of attention it receives compared to all the other sins going on is surprising considering it doesn't directly harm anyone except (possibly in a spiritual way) the two adults consenting. You'd think all of Jesus' commands to take care of the poor would get more "air time" by the conservatives, but somehow it always comes back to talking about homosexuality.