If you play the Demo, you will find out that Inoda actually purchased some very expensive Incense which effect women, like an aphrodosiac.
Inoda had them lit since the very beginning of the date, so Asuna had already to much of this stuff in her blood stream, thus causing her to become very horny.
You also see in the Demo that Asuna is aware of herself being horny, and she tries desperately to leave, until eventually, Inoda's whole planning works and she gets to spend the night there waiting for the storm to calm down, and since she went out in the rain, she had to undress to avoid getting sick.
Also Inoda showing his dick wasn't helping... so she gave in to her desires.