Last weekend we went to a "naked sex party."...
You went to a sex... party. So what you're saying is, they weren't
actually just naked and doing it in, oh, say, McDonald's, or on a train platform, or other places where normal people are usually found. Specifically, they were in a specific place where people with extra-normal preferences hang around having sex. mmkay
In the US, they would be seedy.
In the US they would be seedy. In Germany, they would be seedy, but have better drugs.
In Berlin it is just normal (relatively).
I don't know how long you've lived in Berlin (I guess not long if you still consider yourself a prudish American) but the sort of work you do, coupled with your likely limited experience here, makes me question how qualified you are to speak on behalf of what is "normal" in Berlin.
Unlike a lot of expats in Berlin and most people on this board, I am well-acquainted with the young/alternative native Berliner scene and my relatively well-informed opinion is that it is not at all "normal" to go to sex parties. For that matter it is not even "normal" to lay around naked in parks in the summertime, merely not punishable by law. I think where you're getting mixed up is, mistaking "not something that will get you ostracized from society forever" for "normal".
Germans in general and Berliners in particular do not have the same hangups we do about nudity and to a degree, other people's personal choices, but it doesn't mean they're all super horny sex freaks because they're a bit more tolerant (read: more capable of minding their own business--sometimes). People might work a little less hard here to pretend they are asexual beings, but I'm almost certain that my hometown has waaaay more swingers and fetish clubs than Berlin and it is like 1/4 the size.
The point is, if you go up to a random Berliner and ask them if they would like to accompany you to a sex party the answer is likely to be "uhhhh... no thanks" rather than an American-style response of "Disgusting!! What do you take me for? Vile creature, begone! I will pray for you" or, as you perhaps imagine, the loose and easy Berliner, "well I'm busy that night but a friend of mine was going anyway". There is probably a much larger degree of separation than you have been led to believe, maybe because of the kind of work you do.