It's probably the fact that it's BL specifically that can be toggled on or off. You can't toggle heterosexual content on or off, and you also can't turn GL/GxG/WLW content on or off. But you can for BL. At some point it's a feeling of "either suck it up and go through the BL too" or give everyone options.
In short, homophobia 101, leave the lesbians because it's sexy, but give me an option to disable BL because that shit's fucking disgusting.
(And though I say this, it's natural to not want to see what you aren't attracted to? But wow, we don't get that option, so why should you is how it leaves me feeling, but in general, it'd be best to just give everyone options instead of catering to that in particular or something, you know?)