Sections 1 to 4 of the Sexual Offences Act 2003 set out the offences of rape (s1), assault by penetration (s2), sexual assault (s3), and causing a person to engage in sexual activity (s4).
These require the prosecution to prove that the complainant did not consent and that the defendant did not reasonably believe that the complainant consented.
Section 74 defines consent:
... a person consents if he agrees by choice, and has the freedom and
capacity to make that choice.
Each of sections 1 to 4 state that,
Whether a belief is reasonable is to be determined having regard to
all the circumstances, including any steps [defendant] A has taken to
ascertain whether [complainant] B consents.
The police/prosecutor will ask what steps (if any) the suspect/defendant took to obtain the complainant's consent.
In the case of ambiguity: "I believed the complaint consented because their behaviour was unclear and open to interpretation."
In the case of passivity: "I believed the complainant consented because they did nothing."
Neither case supports the belief that the complainant consented.
In a way, ambiguity seems particularly damning.