I've seen it all. . .
As for demons, unless you could consider Legato and Knives demons, I don't recall any *as such.* There was a character in episode 15 equipped with a teleportation device called the "demon's eye" but it wasn't anything *demonic* in the traditional sense.
Vash and Knives are referred to as "angels" and their ultimate weapons as "angel arms," but that's really more of a sci-fi thing than really demonic in any sense I've ever heard of. . .
as for sex/nudity:
In episode 18, there is some serious fanservice (meaning he is wearing absolutely nothing) on Vash. You don't see anything frontal and it's blurred for the most part aside from one frame.
In episode 23, Wolfwood is shown shirtless and there's a scene implying that he and Milly were, how shall I put this, sharing a lot more than sandwiches and wine. However, this is done incredibly tastefully-it's not like the scene in Eva with the "noises" and both characters shown in some state of total undress. In one frame, the characters are talking, the scene switches elsewhere, then when it comes back to them Wolfwood is staring out the window while Milly is sleeping, fully clothed. I'm not saying it's OK or anything, but I'm saying it was implied and not obscene or anything.
Throughout the series, Vash is implied to be quite the skirtchaser, but also an incredibly unsuccessful one-and usually gets more than put in his place when he tries anything (i.e., he's slapped more than a few times, he gets a door slammed in his face when he attempts to follow someone to her room, etc.). Strangely enough (but really unusual for a lot of anime), in episode 7 he actually refuses two women the town brought for him.
As for profanity, sorry, there is some, although the TV edit has the least, the sub more, and the unedited dub the most. In the TV edit, it's mostly mild-and can even be rather infrequent
dependent on the episode some have more than others-although on occasion a stronger word may be used. In the unedited sub and unedited dub, there's a general uptick in the words (and the "S" word is in the unedited sub and unedited dub while it wasn't in the TV edit), but I don't think the "F" word or curses involving God beyond an occassional "Oh my" are used in even the unedited. (I haven't seen the unedited, I compared the TV edit to the edit lists at Anime News Network.)
Violence is the main issue most people would have with the series: although it's for the most part not gratuitous and considered an act of desperation, hatred, and even sin, there is a
lot of it, sometimes graphic.
Wolfwood is a priest but not a very good one, to say the least. Throughout the story, he's caught between doing good, doing harm, doing harm in the name of good, and so on. Some people think he's actually an interesting character (me, and a lot of others) yet others will see him, his weapon, his attitude, and his portable confessional (
) to be really insulting. The thing is, don't view him as an actual, serious portrayal of Christianity (because he really isn't) and you won't be insulted. You might actually even like him.