let's look at the picture prompt memes as an example.

if you go into the smut one, you're going to see nsfw stuff. if you go into the shipping one, you're, hopefully, less likely to see it.

shipping doesn't automatically equal smut or nsfw content, which is important when and if people are rping around underage people or family or w/e. sometimes it's because the chars are underage and they just want fluffy schmoop shipping. sometimes it's because the players aren't comfortable with graphic smut (think the ftb types). so those types of people will enjoy shipping, but will back the fuck out of any smut memes or not go in them. most smut players don't give a shit and will play all over the fuck. gen only players won't go in either.

so because there is a level of nsfwness in the concept of it, it makes it three different categories. gen (which, btw, isn't always nsfw, but it's just nsfw in the context of porn), shipping (which can be fluffy, schmoopy, kids in puppy love or shy glances), or smut (which is what it says on the tin. you're in their for tits and ass and dick and balls). yes, you can mix and match them, but they are, for the purpose of people playing in them, three separate categories.