I don’t think they necessarily lack a place in fiction but that they’re really hard to carefully pull off and portray. It can be done sparingly or implied at times. But sometimes these are things best left to nonfiction in the form of confessionals, sexology, commentary, autobiographies, anecdotes and the like. These may not be any better but at least there’s a time and place for those.
As for LGBT, it can be really hard to tell if it’s respectful representation or somebody’s paraphilia. Given the issues LGBT people have with cishet people sexualising or objectifying them and ad infinitum, it’s a tricky thing to balance. And at other times, if people aren’t careful, they shouldn’t be doing those things. It’s not necessarily wrong to discuss about sex and violence.
It’s just that sometimes there’s a time and place for those. If you want to discuss about masturbation in a respectful way, look no further than nonfiction (anecdotes, confessionals, scientific studies, autobiographies and sexologies). Same with let’s say STDs and the like. It’s not that they shouldn’t be discussed at all, there’s sometimes just a time and place for those.