Japan actually has two "mature content" ratings when it comes to movies. R15+ and R18+. R15+, which Goblin Slayer is likely closer too, would roughly be somewhere between our PG-13 and R, leaning closer towards R. Keep in mind that in Japanese televised media, there are different standards for what is acceptable. While in Japan TV more or less doesn't have "ratings", the movie rating system (Eirin) is a pretty good standard to go by. There is a lot of crazy stuff that is perfectly acceptable to be shown on TV that you wouldn't even believe.
As a side note, not sure why people have made such a huge deal out of those scenes in the first episode anyway. Berserk did it and people praise it to this day, as they should because it's a fantastic series. Also, keep in mind that sexual content in media is actually a lot more prevalent in Japan than it is in America. As long as they aren't actually showing anything, they can easily get away with a lot. That's why you can't really trust MAL ratings 100% of the time. Even if something looks fun and cute, it could be the craziest thing once you actually start watching.
Personally, I think Goblin Slayer is probably one of the strongest shows this season along with the Slime anime. While I'm not really defending the actions of the goblins as right or sane, but kudos to the studio for going through with it. If you're going to set a tone for a show, you either do it 100% or don't bother.