This would be sexual assault, subject to a mistake of fact defence described below. But the mistake-of-fact defence in relation to complainant identity has only been successful in one case, and has since been rejected on the basis of subsequent Supreme Court case law, so its availability today is doubtful in the absence of reasonable steps to ascertain communicated consent.
As a starting point, this is sexual assault
Assuming the complainant did not actually consent in her mind1, the scenario contains all the elements of sexual assault:
- (i) touching, (ii) the sexual nature of the contact, and (iii) the absence of consent1 (together the actus reus of the offence)
- intention to touch (a mens rea element)
- knowing of, or being reckless of or wilfully blind to, a lack of consent on the part of the person touched (a mens rea element) — it is this element that might be negated by a mistake-of-fact defence
Potential mistake-of-fact defence
There is, however, a mistake-of-fact defence potentially available. While this has not been addressed squarely by the Supreme Court of Canada, the Newfoundland and Labradour Court of Appeal considered "unusual circumstances" somewhat like those in your hypothetical (R. v. Walsh, 2015 NLCA 3). The accused entered the wrong bed in a trailer. He thought he was with his wife but he was not. It was accepted that the complainant did not subjectively consent.
Both the complainant and Mr. Walsh had been drinking earlier, had been asleep, and were half-asleep when the incident occurred. The complainant had her back to Mr. Walsh. When she realized something was wrong, she jumped up, and Mr. Walsh, seeing her, also jumped up. ...
It would have been for the trial judge to determine, based on all the evidence, whether Mr. Walsh had a reasonably held belief that he had returned to his own bed and that the person he touched was his wife.
However, this reasoning also turned on the Court of Appeal accepting that:
If Mr. Walsh had entered his and his wife’s bed, it could be inferred from the marital relationship, in the absence of evidence to the contrary, that the initiation of intimate sexual contact would not require verbal consent, but could be consented to by conduct.
That judgment was from before the Supreme Court of Canada's judgment in Barton (see below) and has not yet been tested against the comments in Barton (quoted below) clarifying the role of prior sexual activities or silence, passivity, or ambiguous conduct, especially in states of semi-consciousness.
In the following sections, I explain if this is instead viewed through the lens of a mistaken belief in communicated consent, post-Barton, it is less likely that the defence would be successful.
When viewed as a mistaken belief in communicated consent, the defence seems less clear
The mistake-of-fact defence could also be cast as a defence of mistaken belief in communicated consent, but this is less clear.
The defence of mistaken belief in communicated consent requires the accused to have taken reasonable steps to ascertain consent "and the reasonableness of those steps must be assessed in light of the circumstances known to the accused at the time" (R. v. Barton, 2019 SCC 33, para. 104). If the circumstances known to the accused did not include the fact that he was with someone other than his wife, this could change what reasonable steps would be required to ascertain consent. But what steps would be sufficient is not clear from the facts of this hypothetical.
In any case, this is the caution that the Supreme Court of Canada gave regarding relying on prior sexual activities, or silence, passivity, or ambiguous conduct in establishing a belief in communicated consent (R. v. Barton, 2019 SCC 33):
in seeking to rely on the complainant’s prior sexual activities in support of a defence of honest but mistaken belief in communicated consent, the accused must be able to explain how and why that evidence informed his honest but mistaken belief that she communicated consent to the sexual activity in question at the time it occurred. For example, in some cases, prior sexual activities may establish legitimate expectations about how consent is communicated between the parties, thereby shaping the accused’s perception of communicated consent to the sexual activity in question at the time it occurred. ... great care must be taken not to slip into impermissible propensity reasoning. The accused cannot rest his defence on the false logic that the complainant’s prior sexual activities, by reason of their sexual nature, made her more likely to have consented to the sexual activity in question, and on this basis he believed she consented.
an accused cannot point to his reliance on the complainant’s silence, passivity, or ambiguous conduct as a reasonable step to ascertain consent... This is a particularly acute issue in the context of unconscious or semi-conscious complainant.
The mistake of fact/identity defence has been rejected post-Barton
One lower Alberta court has rejected the availability of this kind of mistake as a defence. See R v BK, 2020 ABPC 193.
The accused said he "mistook the complainant for his wife and had assumed that if the person had been his wife, then she would have consented to the touching." The judge saw the issue as completely being whether the accused's failure to ascertain communicated consent could be negated by the mistake that the accused thought the complainant was his wife.
The judge did not follow Walsh (described above in relation to the mistake-of-fact defence).
The critical paragraphs:
The accused did not take reasonable steps to ascertain consent, regardless of who he believed the woman on the pull-out couch/bed was. As the Supreme Court stressed in Barton, the defence is not premised on an honest but mistaken belief in consent, but rather on an honest but mistaken belief in communicated consent. In order to make out the defence, the accused must have an honest but mistaken belief that the complainant actually communicated her consent at the time of the sexual touching, whether by words or conduct.
In this case, the accused’s mistake was on the identity of the person he touched. He did not testify that he mistakenly thought she had communicated her consent.
If an accused is relying on a complainant’s prior sexual activities to support the defence, he must be able to explain how and why that evidence informed his honest but mistaken belief that she communicated consent to the sexual activity in question at the time it occurred: Barton at paras 91-94.
There is no evidence in this case that this accused believed the woman on the pull-out couch/bed had communicated her consent at the time of the sexual touching. He conceded that she was turned away from him, apparently sleeping, and that she did not communicate with him in any way before he touched her buttock. He did not take any steps, let alone reasonable steps, to determine if she subjectively consented to her buttock being touched in a sexual manner at the time of the touching.
Rather, as outlined above, his honest but mistaken belief as to her consent is based on a mistake of law.
In the instant case, there simply is no air of reality to the defence of honest but mistaken belief in communicated consent. His failure to take any reasonable steps, as required under s. 273.2 disentitles him from advancing the defence. It was a mistake of law, not fact.
1. Consent for the purpose of sexual assault is only the subjective consent by the complainant. "For the purposes of the actus reus 'consent' means that the complainant in her mind wanted the sexual touching to take place" (R. v. Ewanchuk,  1 SCR 330).