I say probably because summoner is seen as lower class or something.
Since his father probably want something like Sage or Sword master or something high class.
Who said it have to be the other girl?
It's just said non virgin.
The potential ideal situation is that don't let him know the truth that the one seducing him is the Demon Lord till they done it, so the door is open so the trick is out.
She probably trying to sabotage the other but failed at it.