OK, how does one write sex in fiction?
It's so easy to write sex awkwardly (or unintentionally comical)... and you might end up winning the Bad Sex Award. (Go ahead, look. There are no pictures.)
Two challenges of writing sex in fiction:
1. Knowing your audience.
Do you hope to win an all-ages audience? What genre are you writing in?
Readers tend to approach genres with strict genre expectations. Sex will only upset them if they are "surprised" by it.
(Will puberty wizard Harry Potter ever lose his virginity? Probably not. Most of the readers are just not ready for that.)
2. Realism Vs. Romanticism Vs. Pornography.
Real-life consensual sex is glamorous only in the subjective sense -- i.e. for the participants (hopefully). What the participants find "hot", the reader may not. Should you convey what the participants experience, or a give an "objective" seen-from-outside account? You must decide this yourself.
And of course there is non-consensual sex too -- even more difficult to write, since it is offensive and painful on several levels. (This subject is so thorny, I'm not even going to go into it.)
The writer who decides to write about sex, either the implicit or the explicit kind, should have a clear set of objectives:
A) Who is supposed to read this? (People my own age? People of all ages? Men? Women?)
B) Why should the reader want to read this? (To follow the plot? To understand the characters? To get aroused? Or a combination of all three?)
C) Why do I want to write this? (Does it matter to the plot? To the character development? Am I just trying to arouse myself?)
If you don't know why you're writing, if you're not thinking consciously, you will fall into "automatic writing", guided by hormones. The body takes over. The result could end up as pornography (that's "erotica" if you're a woman), and unintentionally comical -- "Bad Sex" writing.
Or you could chicken out, and not mention sex at all. (Chicken.)