There’s no mention of any sexual behavior, just that she has curves and dresses revealingly, and that is not relevant in any way.
Sure, if she’s 12 and acting out sexually, that’s a red flag, but dressing like older girls you see at school or on Instagram or TV is not in any way a flag.
Agree with @kate. The way the LW’s daughter dresses is a red herring the LW mentioned to try and come up with reasons her husband started looking at this kind of porn.
For all we know, the daughter’s shirts look tighter because she’s curvier/more developed. Or she likes wearing leggings. Perfectly normal attire for a 12 year old. But it really doesn’t matter. Clothes = red herring.