First, you make a big thing that you've "never ever" heard of such offences but accept that there could be (and there are) some examples. There is a world of difference between something never happening and something rarely happening (this attitude could even be a factor).
You seem too quick to entirely dismiss trends or tendencies that would make sexual assault by females less common when, all put together, the can explain the disparity.
Men and women are different, that much is undeniable. Differences between the sexes are tend, not definitive of course (e.g. there are very strong women and very weak men but the average man will be stronger than the average woman and the strongest people will be men). Many of these differences - psychological, physical, emotional and social - will influence the statistic tendency of doing anything, including having method, motive and opportunity to commit sexual assault.
Also, you're referring to reported assaults. We know that rape victims in general are often reluctant to admit their attack to anyone, let alone formally report it. The reasons - social stigma, embarrassment, (invalid) feelings of guilt and fear of not being taken seriously - are all going to apply to a male victim and quite possibly much more strongly.
Even if an incident is reported, there could be legal factors. Certainly historically and in some jurisdictions still, the legislation against sexual crimes is focused on male attackers of female victims, making it difficult or even impossible to bring a charge of that nature against a female attacker. Some form of conventional assault charge might be able to be brought instead but that, of course, won't them be recorded as a sexual assault.