Depending on what the OP find sexual I wonder if they're also talking about my comic given it's on the front page right now, has mildly censored nudity, and is considered NC-17 in the US. (A surprising amount of people I know associate the nude form with sex regardless of how it's displayed)
Anyways, I can't have an opinion on what Line should and shouldn't do, it's their website. Also can't have an opinion about how people should raise their kids. Personally I grew up with mature content, watched a lot of horror movies in elementary school, saw anime like Ninja Scroll and Devil Hunter Yohko. My mom was also very honest about how sex worked, so I knew the importance of protection, along with the financial, mental, and physical consequences of childbirth. While liking decent quality art regardless of how sexual it is, I can't say I'm addicted to porn, but not all people function the same or are born with my skepticism.
You guys are being real drama queens though. If you feel something is violating the TOS on Line, or you don't like something, then send them an email. The Discovery Policy section under Inappropriate Contents lists things that go against their policy.
http://www.webtoons.com/en/terms
Also feel free to correct me, but I think Line is a Korean based company? So their age ratings may be different from the US.