These (sex and death) are big issues, among the biggest in our society. They're important to every human being, and anything that's important to human beings is important to a writer. Norman Lindsay once said that the two things you had to include in a book for children were food and fighting. It would be a little foolhardy to say that you should include sex and death in a book for adults, but not many writers exclude them both.
You need to confront sex and death in your writing, and to deal with them in a meaningful way. Perhaps it would be fair to say that the more you've thought about them in your own life, the more you'll be able to write about them. That doesn't mean you've come to terms with either or both of them: some of the most powerful fiction in our culture has been written by people whose attitudes to sex or death could fairly be called neurotic.
The way you write about these things is of course up to you. It hardly needs to be said that writing about them may cause strong reactions from some people. Their reactions will probably tell you more about those people than it will tell you about anything else. Some people are outraged that anyone could dare to write anything that doesn't reflect their own values. But here are some generalisations that may provoke some thought in you:
~ Every relationship has a sexual element
~ For males in our culture, aggression and sex are inextricably linked
~ Fear of sex is rampant in Western society
~ Humans are the only species who know that they are going to die
~ People's fear of death causes them to find all kinds of ways to perpetuate themselves
~ Death gives life it's point
~ Sex and death have a lot in common
Just grappling with provocative concepts like these can help you write with more insight.
Because these are such powerful subjects it's even more important to write about them with restraint. The strongest horse needs the tightest rein. Charles Dickens was able to wallow in sentimentality when he described the death of Little Nell in The Old Curiosity Shop, but those were different times. Fashions change in literature, as in all things. The mood now is for writing that's done with a light touch. When writing about dying, for instance, it's often better to avoid any mention of tears or crying, as it's so hard to write about the act of crying without using stale or sloppy language. 'Rivers of tears gushed down my face' or 'Tears came to my eyes and I sobbed as though my heart would break' or 'My eyes overflowed with tears'. None of these is going to cut it with your readers.
Perhaps oddly, writing about someone trying not to cry tends to be more moving than writing about someone who is crying.
Writing is more effective when you can illuminate elements of life of which people are unconsciously aware; that they haven't consciously noticed. Usually when people write fictitious accounts of sex or death they mention only the excitement (sex) or the grief (death). That's why their accounts ring false. There's much more happening than those two things. When somebody dies, for example, the survivors might feel emotions like anger, fear, confusion, guilt, freedom, relief, shock, happiness. That's a short list; it could easily be extended. It may help you realise why an account of death that only deals in grief will be unsatisfying.
We don't react in obvious and predictable ways. You need to recongise that in your writing.
Marsden, John, Everything I Know About Writing, Pan Macmillan, Sydney Australia, pages 115 - 118