Studies with drosophilia indicate that a gene on the right arm of the third chromosome ironically called "fruitless", determines courtship patterns and sexuality. If there is such a gene in the drosophilia, it is not far fetched to believe that there is a similar compelling genetic factor in human sexuality.
Since 1973, it has been accepted fact that certain compounds found in nature called "endocrine disruptors" play a a huge role in determining gender assignment in fish and other reptiles and also in influencing sexuality in greater cats in the wild. These endocrine disruptors are found in more heavy concentrations in the human food supply due to our use of artificial growth hormones and antibiotics.
Further, twins studies consistently show a pattern of higher rates of homosexuality in identical twins than in fraternal twins, indicating a genetic factor.