I would go by MPAA standards. Mostly, the profanity=nsfw correlation has to do with context and frequency. IMHO, using the F-bomb more than a couple of times in a comic might warrant a nsfw rating. If you want to play it safe, mark it nsfw or mention in the summary that there is adult language. As for the forum posts...well, people will say what people will say. Not everyone shares the same attitude towards curse words. Personally, bad language doesn't bother me, as long as it isn't hateful or degrading towards a person/group of people. My comic is a mystery with comedic undertones, and while there is some mild language(?), it's nothing extremely offensive, so I don't think it requires a nsfw rating. Generally, I think the NSFW warning is reserved for nudity and sexual situations and extreme violence (e.g. Walking Dead and Game of Thrones status).
But that's just my opinion ^.^