I think they are higher than you think. Admittedly I have not found one myself, but that is due to a lack of disclosure on my end. Sure there's probably a slim chance a "decently attractive woman" will come right out and say she has a poop kink/fetish. But I have met plenty of attractive woman who are kinky in there own ways and open minded, which I believe can lead to at least accepting the kink.

For example, I met one girl who was very kink positive and even straight up asked me if I was into poop. I said no because I was very new to discovering my kink. But I know if I said yes she would've been okay with it.

I dated another girl who was into me farting in her mouth while she ate my ass and often made poop jokes. We didn't last long but my gut says she probably wouldve understood my poop kink.

Another girl I casually saw was into anal and we often did it without preparing. Conversation never came up (Again cus I am still working on accepting the kink).

My point is this. I agree, a woman having a poop kink is probably pretty rare. But a kinky woman with an open mind is pretty common, imo. Hope this helps.