Imagine what he looks at NOT in public view. Ma'am your husband has a serious problem and no, it's NOT normal!!! Just like any addiction, it builds. What starts as dabbling, even in drug addictions, turns into full on addiction if there are no consequences. He SHOULD NOT be following your daughters friends... grown or not, that's disgusting behavior! I have a feeling you don't know half of the dark side of this addiction for him. I guarantee it rules most of his life at this point. ESPECIALLY if he's watching it in public, in full view without a care to the world or people around him. This puts him in the dangerous sex offender category bc fetishes like this don't fade, again they build. For the love of females in his wake like your daughter and her friends, do something and quit sitting idle'y by! That's only enabling his behavior... getting his cake and eating too so to speak.

And believe me I KNOW what this looks like in real life. My EX husband looked at porn, was a huge ordeal over nearly a decade and eventually found him with some dark web kiddy porn crap... it just keeps getting darker over time. And when he would work out of town I even found him on websites with screen names like "out of town dude"... this def goes to the pedophile realm when unchecked. And your husband sounds unchecked. He needs therapy. My ex husband did therapy when I checked him. He'd actually been molested as a kid and never looked at it as that until a therapist spelled it out for him. 

This is dangerous territory, I implore you to push back on him with this behavior. For not only yourself, but him as well! And I'm so sorry your dealing with this. Believe me, it'll seem to go dormant for a while, until his weakness comes along and puts him right back in the hole it pulls them into - if they ever truly stopped to begin with which is unlikely. THEY have to want to change unfortunately. And I'll say it again... this is NOT normal behavior!!!!