I cannot speak about feelings towards underaged girls, much less acting on such feelings since though I have my share of issues that has never been one of them.
One thing however is mb and dreams is a difficult subject anyway simply because anyone in their dreams lacks control and what you might attempt and struggle to control while your awake will surfice in dreams, whether this is worries, fears, hopes or even desires.
My own problem is genophobia, that is a severe fear of s/x and indeed my own reactions, and I use mb as a control on my dreams since if I stop, I know myself I will experience extremely frightening, s/xual dreams, and likely mb in my sleep or worse, though for me at least mb is a pretty mechanical and distant process and about the most I will think of is a certain lady's face or at most holding hands since anything more triggers my genophobia.
In fairness I have never made love in my life nor indeed ever had anyone to make love with, and if I did this may be different, however the fact that! even though the thought of anything vaguely to do with s/x causes me to freeze or panic I need to engage in such controls to avoid dreams does indicate something about the subconscious mind and it's relationship to the body.
While I don't think your husband was responsable for his actions in dreams, just the same way a person who is sleep walking and stamps on someone's foot isn't responsable, equally this might indicate that he needs to considder other ways of dealing with and processing his desires. What those are I do not know, since my own issue is so different, (even standard s/xual attraction I find a pretty incomprehensible thing), but maybe it's something he should discuss with his T.