In Clarkson v The Queen; EJA v The Queen  VSCA 157 the Court of Appeal (comprised of five judges) considered the relevance of an unresisting victim to sentencing an adult offender for child sex offences.
Maxwell ACJ, Nettle, Neave, Redlich and Harper JJA [at 3]:
The absolute prohibition on sexual activity with a child is founded on a presumption of harm. The prohibition is intended to protect children from the harm presumed to be caused by premature sexual activity, that is, activity before the age when a child can give meaningful consent. It is for this reason that a child’s consent is more accurately referred to as ‘apparent’ or ‘ostensible’ consent. References to consent in these reasons should be understood as having that connotation.
We have concluded that a child’s consent can never, of itself, be a mitigating factor. That is, proof that the child consented will not of itself differentiate the case for sentencing purposes from one where the child’s consent cannot be established. (Proven absence of consent, on the other hand, significantly increases the seriousness of the offending and the culpability of the offender.
Proof that the child consented is the beginning, rather than the end, of the sentencing court’s enquiry. In assessing the gravity of the offence and the offender’s culpability, the court’s attention will be directed not at consent as such but at the circumstances in which the consent came to be given.
Typically, the giving of the consent will be a reflection of the relationship between the child and the offender. In very many cases, the consent will be seen to reflect a significant age difference and/or power imbalance between offender and victim. In such cases – for example, the consent given by a pupil to her teacher, or by a daughter to her mother’s partner – the circumstances will usually reveal the offender’s abuse of a position of trust or authority, rendering the offence more grave and his culpability greater.
At the other end of the scale, there are exceptional cases – for example, in a relationship between a 15-year-old girl and an 18-year-old boy – where the consent is, relatively speaking, freely given and genuine and a reflection of genuine affection between the two. In such circumstances, as the cases illustrate, the sentencing court is likely to view the offence as less grave and the offender’s culpability as reduced. In such a case, too, the offender may be able to establish, by appropriate evidence, that the victim is not likely to suffer the harm which the law presumes to flow from premature sexual activity.
In short, to ask whether consent is a mitigating factor is to ask the wrong question. It is only when the circumstances in which the consent was given are properly understood that the court can appropriately assess the offender’s conduct and, hence, determine the appropriate sentence.
The Court did acknowledge that an accused should be allowed to attempt to rebut that presumption of harm [at 52], though such an attempt may be fraught with risk:
On ordinary principles, it is open to an offender to seek to demonstrate, to the requisite standard of proof, that the sexual activity in question did not have (or is unlikely to have) the harmful impact on the victim which the law presumes it to have. Put another way, it is open to an offender to lead evidence to rebut the statutory presumption of harm. To the extent that such a submission relied on the consensual nature of the sexual activity, the court would draw on its assessment of the circumstances in which the consent came to be given, in particular the age difference between the offender and the victim, the nature of the relationship between them, and the circumstances in which the sexual activity was initiated.
We think it likely that such an attempt at rebutting the presumption would succeed only in very limited circumstances. For obvious reasons, a statement from the child victim would be unlikely to satisfy the court that no harm had been caused or that there would be no long-term consequences. Independent expert evidence to that effect would ordinarily be essential. Moreover, it would only be in a very clear case that such evidence would warrant a material reduction in sentence. The task of a sentencing court is difficult enough without having to deal with gradations of harm to a child victim, particularly when much of the assessment of harm involves predicting long-term consequences.
So in practice the position is more complicated than just ignoring the attitude of the victim. Normally a court will consider (and is justified in presuming) that a child victim of a sexual offence will suffer negatively as a result. But if the court is satisfied that a particular victim will not suffer that presumed harm, that will be mitigating.