Boring porn cliche. These are probably the most over-done story lines in erotic fiction:
My best friend's mom (dad)...
My sister's (brother's) sleep-over...
The delivery boy (girl)/ repair-man (woman) and the lonely housewife (neglected husband)...
The naughty employee/ boss...
Seduced by the next door neighbor...
Help me! I have morning wood/ an incurable boner and I might die!...
The bad student needs a good grade...
We accidentally saw each other naked. Now we have horny OCD...
There's only one bed at this hotel. What will we do now?...
The random hot girl (boy) all alone at the club/ pool/ gym/ wherever one night...
Desperate for cash...
Caught jerking off, they stayed to watch/participate...
The "special" massage...
Detained by the authorities..
.
We went skinny dipping down at the creek...
The AC broke, so we're nudists now...
Personality-lacking, cookie-cutter NPCs fucking for no apparent reason.
If you're new to writing erotic fiction, by all means take a crack at these stories. Most of us have at one point or another in our 'careers' as writers. Hope you put a unique, original spin on them. But if it's more of the same run of the mill plot lines, be aware we've already seen it a thousand times, and it's going to be pretty difficult to stand out from the crowd.