Double standard implies that it'd be okay if applied to both, doesn't it?

I don't think necessarily, but it would be okay if the standard applied to dildos and romance novels and etc. was applied to yaoi. Though actually some of the sex toys they sell are likely unsafe, and some of the lube causes yeast infections in those prone to them: phthalates (found in jelly toys) and glycerin (found in KY) are bad mojo. So if Amazon actually wanted to protect their customers, they'd actually warn about that stuff. And they certainly wouldn't censor yaoi, which has never caused genital itching as far as I know.