Who said anything about that? These movies all feature actors who willingly took part and got paid for their efforts as part of the professional industry. You won't find anything else here.
I took the OP and thread title to mean "girls doing porn videos and struggling with the anal but still doing the scene". Is that right?
Melissa from Exploited College Girls: http://myhotcollegegirls.com/94/bj.html
The clips don't do this fantastic scene justice (well maybe #2) as Melissa gets her ass absolutely reamed in a long long scene! Lots of closeups on her wincing face, at the end the camera guy shows us how red and sore her asshole is as he teases her about not being able to sit down on the way home. :1orglaugh She's not really in great pain, just wincing and struggling and looking submissive thru the whole scene. Fantastic!