I'm curious, how exactly do you know this? You stated this as fact, inquiring minds want to know.
Well you can take it from a physical standpoint.... he said he has a tiny dick.... if he can barely get in her it will naturally be tough for another dude to get in position and slam her, just google a DP on a porn site and then imagine a tiny dick midget taking up either position on the team, you will see what I mean.
But if you deny that, which you could.... Cersei is a shallow cunt.... she would never fuck a midget..... her hot brother, sure.... but cmon she wont stoop to fucking a dwarf.
Or I could be entirely wrong, maybe she was hoping for a sister she could experiment with..... then she got a midget brother she couldn't fuck.
Cersei is a crazy ass bitch...