As someone who is married to someone who was a model I for one can say I have occaisionally done it.
The line is always are you forcing her or is she willing?
Let's be honest and say that some girls do see it as a way of getting into a shooters good books and will fuck him to get more work. What they don't realise is the enormous costs we have for shooting and unless she sells we don't shoot her. However having consenting sex with a model brings you closer. She is looking into the lens of a guy/girl she is fucking.
Girls get horny when they are doing this work, turning them down can be a big put down and has to be done carefully.
The image that we force girls into having sex with us to get the work is stupid and shows how little those who believe it know about this business and shooting. Force a girl to have sex with you and you have an enemy, she will not look so good on camera. Her work will simply not sell as well. You also fucked her so she has to do less to justify getting the work done.
You will also get a very bad reputation and will lose out on models, bad news travels.
The truth is most good shooters have a personality/character girls like and sleeping with some of them is no problem.