Scientists injected testosterone into pregnant guinea pigs at a specific time of development and all of the female offspring were born with ambiguous genitalia. These offspring were then put into heat and instead of arching their back they mounted as much as the male offspring. The females exposed to testosterone prenatally then not only developed ambiguous genitalia but also had their brains change to make them mount instead of arch. If you inject guinea pigs with testosterone as adults, there is no effect at all.
We scanned the brains of rodents to try to find a difference between males and females and surprisingly we found something. There is section of the brain right where the optic nerves cross that has been labeled the sexually dimorphic nucleus, or SDN. This section in the male rodent is 5 times larger than females and in our own species its 2 times larger. The SDN size depends on hormone exposure in utero...meaning how ever much testosterone is exposed to the SDN in utero will determine how big it gets, thus determining how masculinized the brain will be.
So, when we gave testosterone to pregnant rats, the female offspring were born with male sized SDNs, causing them to act like males. When we gave pregnant rats testosterone early in the pregnancy, the female offspring were born with masculine genitalia but acted like females. When we gave the pregnant rats testosterone late in the pregnancy the females were born with female genitalia, but acted like males.
What does all this mean?
All offspring start out feminized but when a larger amount of testosterone is released, it causes females to become males who then develop testes and a large SDN. Without the large exposure to testosterone, the body and mind stay feminized.
If you expose a female at a certain time of development to testosterone, her SDN becomes large, causing her to have homosexual behaviors.
If a female receives enough testosterone in the beginning to develop testes but not enough to enlarge their SDN, the male will exhibit homosexual behavior.
Homosexuality is natural.