I think you need to work out what your views are on pornography first, as your fella's collection of %&*$# seems to be getting tangled up with your worries over how he views women and if he possibly views your daughter and her friends in the same way? I think getting these things confused will end up in massive trust issues and heartache.
Aside from your worries about your daughter and her friends, what are your views on %&*$#? Have you been happy for it to be part of your previous relationships? You mention that your bloke is open about his use of %&*$# which is a good sign. Would you rather he viewed it with you or without you? Expecting a man to stop using %&*$# is a bit unrealistic - this is highly unlikely to happen and will force him to be secretive about his use of it and then drive a wedge between you, thus causing a cat and mouse game, disappointment, and broken promises.
Now to the content of the %&*$# - do you find the content of the %&*$# disturbing or the girls featured in the %&*$#? The fact that the %&*$# centres on a specific area - stockings, suspenders and heels suggests he is into legs and feet and lingerie. I can understand this, as I have a real thing about stockings, suspenders and heels myself. Is it possible that he watches %&*$# to satisfy this aspect of his desire and he feels that you might judge him in some way? It is likely he is focusing on this aspect of the %&*$# (stockings, suspenders, heels, feet, legs) rather than the girls themselves. Would you be prepared to discuss this with him and maybe introduce these things into your sex life? Agent Provocateur do some amazing vintage seamed stockings, and a man who worships you in a pair of stockings and heels will be prepared to keep you in beautiful shoes!
Now for views on women and your worries regarding your daughter and her friends. I think that a lot of men compartmentalise their views on women from watching %&*$#. I obviously don't know your partner, but I think it is highly unlikely that he views your daughter in an inappropriate way. It is very likely that he hasn't even made a link between the ages of the girls in the %&*$# and the ages of your daughter and her friends. From what you have posted, it seems that he watches %&*$# to allow himself access to things he fantasises about but feels unable to act out? I have often fancied letting my inner Katy Perry out, but because I have been in relationships and I am always completely faithful, watching girls in %&*$# is my way of expressing that part of my sexuality in a safe way. This might be true of your partner?
Regardless of any of this, it is important to work out what YOUR views are on %&*$# and what your boundaries are. Expecting him to not watch %&*$# is unlikely to work - if you are unable to accept his use of %&*$#, then it would be better to end things.
Hope this helps! Hugs, Boo xx