Where do you think is the balance of eroticism? In the act or in the circumstances?

[WhiteTailDarkTip]

Circumstances. I almost get bored once it gets to the actual sex.

Yep. I can get to a point and think ‘oh yeah, this is supposed to be erotica’ and find it hard to, um, insert (oh, matron!) the necessary descriptors.
The sex has to matter for the story. It’s possible to write good porn with minimal incidental detail but it’s not a common skill set.