If he wants some extra lube while he fucks her he will pull his dick out and you will be there with an open mouth, ready to serve.
It may be smelly or dirty but no dick is too good for you. I don't know how well your ex-girlfriend looks after her anal passage but I am sure his cock won't smell good after it has been jammed up her ass.
You are at the bottom of the food chain though, and sissy girls like you have got to eat something.