Ask God about it.

Touya thought a bit longer, before he had an idea. "Hey, uh... God? You there? You listening?" He then paused, and face palmed. "Right, forgot." He picked up his smartphone. He checked the call history, and luckily, the call he got from God when he first arrived was still there. "Let's see if I can call him," he mused, hitting the Call button.

Almost immediately, the call picked up. "Well hello Touya!" the amicable old god said cheerfully. "Have you adjusted well to the world so far?"

"Well enough," Touya grinned. "I actually have a question about something."

"Is it about that notebook that almost hit you in the head?" God asked.

"Actually, yes. Is this some other gift you gave me?"

"Not me per say. Let's just say there are some other gods and goddesses, some local, some... Not. The Sex Note belongs to gods of the latter category, and we got a visitor of that persuasion. She likes to drop a copy of them into mortal worlds from time to time. I did ask her to throw it in your direction though. Felt it was both better in your hands, plus I still feel a bit bad."

"Was she actually trying to hit me in the head with it, or was that just an accidental near miss?"

"Apparently, whatever power guides that particular Sex Note down to earth, besides gravity, has an interesting sense of humor. Almost every time she drops it, it does clobber whoever ends up getting it. She was surprised and a bit amused to see you dodge it."

"So it's fine that I have it then?" Touya confirmed.

"Yes! So just use it already before I get bored and give it to someone else!" a female voice shouted from the other end, causing Touya to jump, and comically fumble with his smart phone. "Just don't go ruining people's lives with it, and make sure they actually like what you do, and they'll be happier for it!"

"... that's her, isn't it?" Touya asked, recovering his composure.

"Yes, she's quite a character," God chuckled. "But it's like she said, you're free to use it as you please, just don't ruin any lives with it. Unless of course they have it coming, but I'll leave that up to your best judgement. I generally prefer to let mortals make their own decisions as long as they don't screw up their worlds."

"Alright then," Touya nodded.

"Talk to you later," God said, but just before he hung up, the female voice shouted, "Make sure it's entertaining! I'll be paying you a visit at some point when I feel like it, just make sure it's not to take back the Note, okay?"

"...okay?" Touya asked, before he was hung up on. "...well, life really is going to be interesting around here... Aw crap, I should have asked about Ishen while I was at it, and whether pretending not be from there was a good cover... Oh well..."

He then put the quill to the Sex Note, and first wrote, "Touya Mochizuki will be able to control whether or not he impregnates a girl when he ejaculates in them. Furthermore, he will be immune to any STDs, ans be unable to transmit them." That should eliminate most potential complications to having a somewhat carnal lifestyle with this thing... Now, who to try it out on first? He had learned that the Innkeeper's name was Mika, no surname for some reason... Or perhaps Elize and Linze? Hmmm...