Swearing can be forced if it's not something that seems like what a character would say. So generally, it depends on how the character acts and speaks, but yes, it can sometimes be forced. For those cases, I can be turned off my swearing, but it doesn't overall. I mean, I swear a lot in reality, so it doesn't bother me in the slightest. Actually, I find a little swearing can add to the realism in the story if the characters fit, like if they're a ragtag bunch of criminals.
In some cases, swearing can set a somber tone, but it can also be used to display comedy. Generally, it depends on how it's used, just as all other words, really. But swear words can add to the tone of your story. However, I wouldn't rely on them. Although, it's worth mentioning that you don't want to swear too much in your prose; it should mainly be saved for dialogue unless it's in a first-person narrative and the narrator is a swearer. If you can make it work, though, then go for it. It does set a tone depending on how you use them, so it'll be interesting to read.
And I think if there's an f-bomb, it automatically receives an M rating for movies. I don't exactly agree with that unless it's excessively used, so I think you'll be safe with a T rating if you use the f-bomb sporadically and warn readers beforehand.