Post divorce: It's Lestrade and [male partner of your choice]'s first time together. Only problem is, Greg's never been with a man before.

Basically Greg tries really hard to pretend to be more experienced than he is, because he's afraid the other guy wouldn't want him any more if he knew Greg's only ever been with women before. The other guy figures it out anyway, and proceeds to make the experience memorable in all the right ways.