Im gonna weigh in on this. Firstly though Id like to address why this thread got locked. Despite how it initially seems, this isn't being locked to ignore criticism. If that were the case we could hide the thread completely, delete it even. The reason it's been locked is because an open thread aimed at criticising a sites staff (any site, by the way) and decisions almost always goes downhill pretty quickly. Members who feel upset at a mods decision use it as a place to, excuse the metaphor, shit on the sites staff members. In addition to being something we dont exactly appreciate, some users who are happy with how the site is run will inevitably come to the defence of the staff. Opinions clash, people fight ... you can see where this is going. We would much prefer people to post criticism in a more focused way. For example: Instead of: "What problems do you have with MLP Forums?" Try: "Excessive vulgarity policy; could we change it?" As you can see, the first thread invites people to talk about whatever they so choose...including personal grievances with decisions against them. The other thread however, invites people to discuss a specific policy we have. The OP will likely have talked about what they feel needs changing and doesn't really leave room for people to come in (outside of them going completely off topic) and turn it into something abusive. In addition to that, if you wish to vent frustration about a decision you are always free to contact a moderator and we'd be happy to talk to you about it.
Now to respond to your specific issues.
Nipples is fine. You wont get a thread removed because you said nipple (unless your thread is literally nothing but the word nipple) , that's not even a typically sexual body part.
If you've created a thread about masturbation and it turned into a "Oh I love to pet the snake four times a day" kind of thread then it will probably get removed because we have younger members here. We're fine with people discussing health topics and referring to masturbation from the context of a philosophical debate but if a thread turns/is turning into a rather 18+ discussion then it doesn't have a place here.
The second topic you talked about is fine because it isn't an inherently 18+ discussion. Talking about peoples approach to discussing sex is a genuine discussion that doesn't particularly invite people to get into the nitty gritty details about their sex life, in fact it really has nothing to do with peoples sex life. That's why its fine.
Honestly this really boils down to common sense. Fuck and shit are used practically all the time by people of all walks of life. Everyone and their grandmother has heard these words. Whereas there are other 4-letter words that you don't hear often or that are decidedly more crude. Im not going to say which ones but its fairly obvious, especially when those words aren't typically versatile in their use. Fuck can be used as a verb, both transitive and intransitive, a noun, an adverb, a conjunction and an interjection. How many other 4-letter words do that? That's why its one of the most acceptable crude words to say online. Most of the other words (4 letters or not) are used to describe other individuals in a demeaning light or to describe parts of the body in a very crass way. Common sense dictates which ones are relatively fine to use.
The way our moderation system works is designed in such a way that it requires multiple moderators to agree on a course of action before it takes place (warnings, suspensions, bans, so on). That way we can be reasonably sure no single moderator is treating people according to their own personal code. It doesn't make our system perfect, but it does attempt to minimise any sort of huge disparity in how we respond. So yeah, we do appreciate you bringing forth your criticism, just try to post it in such a way that it doesn't invite all manner of arguments about whatever topics people wish to bring up. It doesn't end well.