We've discussed use of vulgar language in fiction on this forum a number of times, e.g. Swearing - Censor, allude, or include? and Swearing in a book, within a context. Too offensive?
To summarize my opinion briefly: Some number of people are offended by vulgar language and won't read a book that has too much of it -- what constitutes "too much" varying from person to person. Very few people will throw a book away because it doesn't contain enough vulgar language. I've never heard anyone say, "That could have been a good book, but it just didn't have enough swear words in it." So you'll lose readers by adding swear words, while it's very unlikely that you'll gain any. It's a risk with little reward.
That said, sure, if you make your language too mild, it can be unbelievable or sound childish. Like I said on one of those other questions, if you have a story about vicious, murdering drug dealers, and at one point the gang leader gets angry at one of his henchman, and he yells, "You silly person! Why are you so mean to me?" ... that doesn't sound very believable. In real life he'd probably be yelling a string of swear words. But still, if you had him say, "You worthless moron! I'll kill you for this!" that doesn't sound particularly unbelievable. Yeah, in real life there'd probably be an F-word or two in there, but as written it sounds plausible.
If people are reading a book about rape and torture, perhaps they are the sort of people who have a higher "tolerance" for vulgarity than most. But not necessarily. Agatha Christie mysteries and "Murder She Wrote" are all about murder, surely the most horrible evil thing a person can do, and yet the audience for these stories tends to be pretty refined. I would be quite surprised to hear a character on "Murder She Wrote" use profanity, and I suspect most of the audience would be offended by it.
So in a nutshell, my advice is: Use a minimum of vulgar or explicit language. Don't go overboard. In a novel intended for adults, I wouldn't write, "Bob went poo-poo." But I would say, "Bob went to the bathroom" rather than "Bob took a s--t".