You've asked a lot of questions there!
How can you tell if someone is real? I think by the depth and detailed nature of the fantasy really. I read a story once by a guy who ironed his cock; not really my thing, but he was explaining what settings he'd have the iron on for, how long he'd leave it before er.. ironing and so on. It led me to believe that yeah, this guy has actually done this thing - he knows what he's talking about! There's also the point that some guys are just going to want to talk dirty, regardless of the fantasy, and I think at that point the person listening needs to decide if they want to get off on this as well. Of course, you're not necessarily going to find this out until you've spent some time talking to him, but if he's clear on the fantasy and can give a lot of detail, then I'd say that yes, he's for real and it's a real fantasy, to the extent that a fantasy, being fictional, ever can be 'real' in that sense.
A more difficult question is 'what is acceptable and what isn't'. In the final analysis I guess that's up to the Mods to decide, based on whatever criteria they have. So, speaking personally, whatever *fantasy* someone has is acceptable - I really don't care how strange it happens to be. I might not like it or understand it, but having it, well that's ok. There is however a line between having a fantasy and wanting to act it out, and there of course I would have problems. If someone wants to have a non-consensual fantasy that's fine, but if that leads them to go out and, for example, someone that most certainly is not fine! So, if it's only a fantasy, yes, I'd say it's fine to have it.
Do I want to read about them all? Probably not, no. There's a lot of stuff that doesn't interest me at all, so I'd probably skip it. However, it depends also on why they are posting. If it's just to get off on writing it down and having people read it, I can manage without it. If they're posting about it because they're worried or concerned or just want to ask 'is this normal' then I may well spend some time with the posting.
Is it acceptable to write about it here? This is a difficult one, but I'd say that, for the sake of the site as a whole, it probably isn't always acceptable. If someone is writing about having sex with an animal for example, not only is it illegal in the UK, it's going to upset a lot of people who would not necessarily expect a site that covers the swinging/dogging scene to have that sort of material on it. It's going to be off topic as much as anything else, and I've seen threads stopped because of just that reason, and I think that's entirely acceptable. I do however think that it's worthwhile if, when such a thread is stopped it's made clear that it's stopped because the subject matter is not appropriate for the forum, rather than it's been stopped because the subject matter is sick or depraved or not normal for example.
I don't think we can get into the game of 'this is normal and this is not' because quite frankly it's down to the individual to decide for themselves. What *I* do is normal, because I do it, and so do others here. On the other hand, people could equally use the same arguement to say that I'm not normal, because they don't do it, and neither do any of their friends. I would draw an absolute line however against talking about illegal activities, such as sex with the underaged, or bestality simply because it IS illegal in the UK, and to have such discussions may well attract people with no interest in swinging or dogging or journalists. I do think there IS such a thing as bad publicity.
So, in summary, I don't think there's anything wrong with having ANY fantasy, as long as the person having it does not take it further if it's illegal or non-consensual. I also don't have any problem with people talking about them either, as long as it's appropriate for the general subject matter of the forum. Many of the fetishes that you've mentioned are not, in my opinion, appropriate for the forum here for just that reason, and not because they're wrong or sick or abnormal.
But... that's just my opinion!
Phil.