Is raping a male arahant or a non-arahant less immoral/less serious than raping a female arahant?

I've read the following passage and I wonder if it really implies what it seems to imply. > The gravity of the offence is determined by the degree of lust motivating the action and the qualities of the person against whom the transgression is committed. When the lust is very strong, even incest and rape can be committed, the most serious being the rape of a female Arahant. http://www.urbandharma.org/pdf/PDF_BuddhismCourse/11_%20Five%20Precepts.pdf If it does, why is raping a non-arahant less serious? After all, the lust is equally harmful in both cases, but in the non-arahant scenario the rape is additionally harmful to the psyche of the victim, which wouldn't be the case for an arahant. So why the act that has more negative consequences is considered less serious? And why are men and women treated unequally in this regard?