Say, a 40 yo man with a 31 yo woman is "weird"? because that's what you're saying. Both are adults now, if you find it strange YOU are a weirdo, not the author.
@Neltharn It's not a set amount of years that is creepy, it's in respect to their ages that it is. He's older by a lot for an 18 year old, 9 years is half her lifespan. Plus, the day you turn 18 you're not suddenly a super mature adult, your brain is still developing.
It is clear a lot of you haven't heard of the "half your age plus seven" rule. Generally, its not as creepy when the partner is at least half the age of the older one plus seven. If he's 27, that means it'd probably be less creepy if she was closer to 20 or 21 instead of 18.
Edit: Is anyone really surprised by this development though? The story has been building to this for all of its 10 chapters so far.