I don't think reducing porn use would hurt the ISPs. In fact I think it would help them - because streaming video is such a bandwidth hog. I haven't ever actually paid for online porn. The last time I spent and money on porn must have been back in the nineties when I rented a VHS tape at a video rental store. So I'm not sure where the money is in all of this. I think ISPs are probably reluctant to regulate porn because they are afraid that it will then be their responsibility to do the filtering.
There is no mainstream populist movement at this point in the U.S. advocating for more regulation of porn. Traditionally, the anti-porn faction has been composed of evangelicals and religious fundamentalists. Almost as a Pavlovian response, the progressive left has rejected porn regulation as an issue to rally around. After all, masturbation is healthy, no need to be ashamed of it, don't be uptight about sex, wink, wink, etc. etc.
However, progressives tend to give a lot of weight to science and research. If research indicates that video porn use causes long-lasting damage, especially to kids, then there may be an opportunity to make this an issue of concern. A smart, independent-thinking politician might see an opportunity to embrace an issue that could potentially have wide-ranging, bipartisan support from both sides of the isle.