Humans are sexual beings. Noticing, flirting, and kissing usually happen before the act of making love. All these precursors will usually give more insight into a character than graphic descriptions of carnal groping (unless, of course, your genre is erotica). The level of detail needed will depend on why the sex scene is in your work. What is the objective? Character development? Is a sexual encounter crucial to your plot? If so, how much should you reveal?
Below are two very different examples of sex scenes, the first illustrates the use of sex in character development, the second in plotting.
In Pearl S. Buck’s The Good Earth, she handled the issue of her aging (seventy year old man) protagonist’s lust for a young girl (Pear Blossom) as follows:
“When it was done, this love of his age astonished him more than any of his lusts before, for with all his love for Pear Blossom he did not seize upon her as he had seized upon others he had known.”
Contrast this to Stephen King’s Dark Tower: The Gunslinger when Roland couples with the oracle to gain insight into his fate:
“The shadow swung over him… There was a sudden ecstasy broken only by a galaxy of pain as faint and bright as ancient stars gone red with collapse.”
Note that in both of these examples the writer was true to the world the character lives in. My rule of thumb about sexual description is to write only what is needed to further the story. With sex, more often than not, less is more. Just like with violence, it’s best to avoid slipping into the realm of gratuitous.