A few points from a male perspective:

First of all, yes, red flags all over, you should be seriously concerned. And when you think about "American Beauty", you should look a little closer than that and that is your very own daughter!

Now, I believe to read between the lines that you are not only wondering what to do about it, but also trying to understand what's going on there. I do have a theory about this which *might* be an explanation, although for sure not an excuse.

I believe it is relatively normal for a teenage boy in puberty to fall in love with some girls in school and also have hot phantasies about them as the testosterone kicks in. It is normal to start "dating" then even if it means only this classical "Will you be my date (Yes/No/Maybe)". And if she checks "yes", usually a shy quick kiss is about as far as it gets when you are 14. But you do have phantasies for sure, for example how she looks like naked. This is about what happened to me at that age. Now, luckily for me, I did have some "success" at that age and so I grew out of that just fine when I turned 18, 20, 25 years.

You say that he was shy and nerdy. So, chances are that he got badly humiliated by the girls in his class. What maybe happened is that the few times he dared to ask a girl out, that she didn't say "Umm well, you're a really nice guy and everything, but ... you're not my style, so sorry, no." But he rather got to hear like "OMG, hell will freeze over before I'd go out with such a disgusting freak like you, get lost!". Or something to that extent. I believe that this can be really traumatizing at this age. And according to some theories if trauma happens then an inner part of you gets stuck then and there. A part of you (or maybe an instance of you) remains there at this very age. Have you ever seen an adult having a temper tantrum or showing a really childish behavior and you thought to yourself "Geez, he/she is behaving like a 6 year old"? Well, it could really be that this person had experienced trauma at the age of 6 and gets kicked back by some trigger into this very age. I know today that exactly this happened to me, that for a long time I was trying to make up for that what had happened to me at young age, although not in any sexual way. Or for example there are young women who keep on taking up with 20-30 years older men and just aren’t able to learn from experience. They do it over and over again. The reason for that she might be still seeking her father’s love, attention or acceptance that she never had when she was a child or adolescent. So, it is a possibility that an inner part of your husband still is a 15 year old boy trying to get a date with a 15 year old girl.
That being said, even if this is the case with him, he is an adult now and he is fully responsible for what he is doing. He has to be aware that this is very, very wrong. He has to understand this himself and seek help. Which is possible to find and get over with. If he refuses to, you’ll have to get out sooner or later as hard as it might be.