It´s a turn-off to see a girl in a cramped position. These moments tell me the girl doesn´t like/enjoy the anal sex. There is not any passion or lustfulness.
It´s like: "I don´t want it. I´ll will be happy when all this shit is over and I get my money!"
Please avoid or cut out these moments. I don´t think it´s a problem to tell an actress to stick her ass out and keep a sexy body posture during the whole scene. There are dozens great examples how to do it right.