Sometimes I kind of managed to write a sex scene while not inspired.
Suprisingly enough, it came out, well... uninspired.
It's like sometimes I can't wait to get to the sex scene. In my mind, it all stems from there, and I have actual problems in reaching it logically by moving in the plot I was already weaving, and often find myself so engrossed with it that I start writing it down before I have managed to lay down the path to it.
This more than once resulted in having to let that scene sit there unused for month or even years.
Sometimes it's the other way around. Maybe I had a few ideas back when I started that line or that story, but I restrained myself, kept my head on working on the set up, maybe just jotted a few notes down, toiling my way up to the fun part and then, poof, everything has vanished from my brain or suddenly feels silly. And for the life of me, as much as I try, I can't manage to remember why I was so eager to get to that point anymore.
And then I try to reignite the spark, write a few horrible drafts, but yeah, I think there's nothing worst that writing a sex scene with your engine turned off.