I know, I've been there!
Will they be naughty enough? Are they real enough? It's an erotic story so they need to have sex quickly in the storyline but that's not realistic? Do people really have sex like this? and more!
It's a tiring process and one that you should really try to avoid. Because it will drive you mad and the key to writing is writing.
I am not, no not! saying you shouldn't outline your characters and really understand them but don't worry about the sex side. Yes people do have sex and yes your readers want to read about sex. It's what they are expecting in your books.
The point of fiction is that it is made up, imagined by you so you can do and say anything you want.
One of my best characters, Gill The Filthy Milf gets sex all the time. I mean if she was real she would be like a porn star. And that series is one of my best selling. So that tells you what people want right?
When you read about her you get just a little showing of her life. A window that you spy into. I don't talk about all the other episodes of her life. The mundane. Why? Because I write erotic fiction and that's what people are paying for.
Let your desires go wild and see what you get.