Having skimmed the raws for the next chapter: this is a reason that's not apparent quite yet.
I have not seen the next chapter (it's already out as of this post, of course), but what you said is true--and alluded to by the story itself.
The locale, being the center of power in that region (if I recall correctly) and home to the upper (if not upper-most, which is almost certainly the case) echelons of society, was--in this very chapter--none-too-subtly implied to be corrupt. Thus, one can expect every sort of corruption that potentially exists in the context of political power--including the sexual. What's more, Sakura happened upon one of those servant girls' orgies and was discovered; the discoverer did
not appear inclined to allow Sakura to get away with
just a peep show--it looked like she was going to make Sakura get a firsthand account of the action (and, also if I recall correctly, Sakura's refusal likely wouldn't have meant anything good for her). Sakura did get away, though.
And since Sakura did, it seems the girls showing up to "initiate" her are just there to collect on her debt. Whether or not the woman who found Sakura would have kept silent (strongly doubt she would), the girls probably would have shown up anyway. They have their own political structure and system, after all.
That said, the monster running around is (from what I remember) targeting the servant girls. It (and it's visibly at least feminine in form) has been shown to have a focused interest in the blood of its victims, stopping to savor their blood. This is clearly inspired by Elizabeth Bathory, a noble accused of killing many women; tales hold that she bathed in their blood--specifically that of virgins--in the hopes of maintaining her own youth. The monster is as withered as an undead. And like
@amamiya426 said, the only males one can imagine being allowed to visit those girls; since the servants may or may not be virgins, I don't think the monster will discriminate, however.
