Because, shhhfastasleep, despite the prodigious application of wrinkle-smoothing creams, I am not a young girl. I don't know what age group you mean by young girl but I'm guessing that this is anywhere between 12-17ish? Much of that sample group would not be able to consent to sex - any sex - in the first place. I can.
If I say that I don't like something, it might be as benign as curly hair (on me, before anybody leaps on that). I do not like having curly hair. It snags and tangles and never looks tidy - so I straighten it. My husband very much likes my curly hair so sometimes, for shits and giggles, I'll let it stay curly for his delight. I don't like it myself but I'll let him twirl his fingers into my ringlets (that sounds dreadful), but I won't be happy until it's straight again. He often does that for me if I'll sit still
To me, it's very much the same thing. Anal sex didn't hurt me, it just felt odd and did nothing for me. And I gave consent. So whilst it doesn't do it for me and I can't say that I like it, it's on my list of 'meh'.
I think perhaps your question/this issue rather comes down to interpretation of "I don't like it" as a response. For some people this statement will be an "absolutely NOT, under no circumstances", but for some people (including myself) it's a "Really? Oh go on then".
The confusion lies in the fact that for myself, I would say "I don't like it" for mushrooms in the first instance and for curly hair and anal sex in the second.
Does that long-winded explanation help?