Because the people and institutions who raised me had a warped view of sexual morality, where the only thing that mattered in deciding if an act was moral or not was if they say that their god approved.
This is why they're always freaking out and making disingenuous comparisons, like "if we let gay people get married then cats and dogs will have sex with babies!" Or something like that. Because God doesn't approve of gay marriage, and God also doesn't approve of your pets and offspring having orgies. If we start doing one thing that God disapproves of, then what's to stop us from doing the other!?
Worse, whenever they talked about how other people see morality -- a word that's been forever tainted for me because of them -- they either lied about them or poisoned the well so I wouldn't look any deeper. They said over and over again that other people based their morals on what's "popular" and "fashionable," and they openly ridiculed the idea of thinking for yourself or doing what feels right. Following your conscience was okay, but only if it agreed with them.
That's why I keep second-guessing myself, trying to figure out the "right" answer to questions of personal preference like what operating system to use. I have trouble seeing "because I want to and it doesn't hurt anyone" as a valid reason to do anything. And I'm scared that if I face the fact that I actually want to have sex with people, it's going to lead to my eating babies.
I've realized that's not the case, ever since I saw this pointed out on Love, Joy, Feminism. But it's hard to convince myself of that.