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