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?
The passage doesn't imply that at all. It's just that homosexual misbehavior or other abnormal ways of breaking the 3rd precept are not discussed here. 

Gravity of the offence also depends on the purity of the being who's subjected to it. Not on how he/she deals with it. It's similar to how merits of giving dependent on the virtuousness of the recipient. Not on how much he/she craves for what is given.