Look, you're the one trying to make some excuse out of an act that any teen should know about. By the time your 13, you should already have an understanding of what sexual activity is, whether you've experienced it or not. Any teen should know what sex is and, assuming they had a proper education, should have at least a basic idea of what genitalia looks like, from either gender. I know many schools go into some pretty distinctive details of how sexuality works within the body.
TEEN is at an age where sexuality becomes a very real thing. The image of a penis, vagina, whatever is only a natural progression of the understanding of sexuality for them. You, like so many others on here, seem to think Teens are a little children who need to have their eyes averted from the real world.
As IdiotWithPencil said, if you have such a problem with it, then its up to you as the authority to prevent the child you're with from experience something you think is too much for them.
But there's not point to that. If the vast majority of LINE"s material is appropriate for TEEN, that's simply the rating they're going to give. One mature series, much less just an episode (though I'm sure there's more) isn't going to cause LINE to completely change up their rating just to fit that one side.
The vast majority of series on on LINE are meant for TEENS and even younger ages than that. TEEN expresses the best point for the site for gaining potential readers. It would seem clear to me that LINE doesn't want to be affiliated as a strictly mature site. So they place themselves as a TEEN rating which better fits them while having mature filters for anything that might go beyond this.
In the end, LINE's going to promote their rating system, in this case for the App it seems, in a way that best fits the criteria of their overall material.
And I'll say it again, who cares? Teens have probably seen far wore than what has appeared on LINE. I mean, it's easy enough to get on a porn site with some very explicit sexual conduct, zero censorship and not one bit of filter on it. The worst of LINE comes off as playful tickling in comparison.
Let your teen grow up, and see the real world and then, if you think its too much, then you who have the power of responsibility can handle it. LINE has basically done their part. They've promoted their series in a way that they want to advertise to a certain demographic and then they've added essentially some editions to it so that it can express better criteria.