But to be clear, my current and all previous partners have assumed all forms of consent unless I’ve verbalised or somehow otherwise made clear that I’m not interested at the point of them attempting it.
How very fortunate for you then that you are an outlier who sees no difference between vaginal and anal sex.
And how very fortunate indeed that you conveniently have always had the opportunity to move away, push them away, say no, or otherwise indicate no if you didn't want to have anal sex on that occasion before you were penetrated.
Unfortunately the op didn't get that opportunity; being held tight from behind, being sleepy, under the influence of a sedative, and believing her new bf was only intending to have vaginal sex. A reasonable assumption.
(And had a typical shock and freeze response when he penetrated her anally without checking consent.
And was questioning herself over whether it could be a mistake).
She wasn't given the opportunity - which is why explicit consent is crucial, and entirely reasonable to expect.
Especially for potentially painful, potentially damaging penetrative sex in an orifice not evolved for sex, that many women are not interested in having.
Any decent person knows that, implicitly.
Just as the men who've posted on this thread have expressed.
(How very fortunate for you that have also been lucky enough not to have encountered a man who likes to enact violence and degradation as part of his sex life, and who started it - too suddenly and quickly - for you to say no). Unfortunately other women aren't as lucky as you on that front either.
Funny how you keep insisting you're not giving any opinions or commenting on anything, but repeatedly refer to assumed consent in the context of this disturbing and sad scenario.