I would say NO ... mainly because you are even asking.
I have to agree with the previous poster on the fact that since you are asking, you are obviously uncomfortable and therefore perhaps discontinuing this might be the best for you if not for all involved. That said, I live in Japan and here it is quite common for fathers (and mothers alike) to continue showering and bathing with their opposite sex children until the later years
Well, my son and daughter showered with their Dad up until a few days ago when our 5 year old daughter started kindergarten. He feels he shouldn't shower with her now that she's in school. He is 24 though, not that age makes a difference.
If his pornography problem bothers you and him showering with them also does I would say it's time to put an end to it.
Agreeing with the other posters.
The porn addiction and the bathing with his daughters don't have to have anything to do with each other to both make you uncomfortable. It is just not the norm in American society for school aged children to be bathing with their parents, regardless if its moms doing it with their dd or dads with their ds. Unfortunately it is going to open your husband up to questions the moment your 5 yo mentions it to a friend or at school.
And don't take this the wrong way but to me and I'm sure to your daughters, it doesn't matter how they were conceived or who provided the sperm, he's their dad, even if he was a step-dad, he's their dad. I don't think that makes your situation different from anyone else in the same position -- concern over a dad still bathing with his kids. If there was/is a problem, how they were conceived isn't going to change my response or I think anyone else's. In fact I would be alarmed if he wasn't their father. Instead, I find myself thinking that it is something he needs to stop doing, more for your older dd's comfort (and most likely his comfort soon, because it can be embarrassing answer questions about differences in boy's and girl's bodies when your dressed forget about being naked in the bath or shower), than because he is doing something "wrong". And that might be the way to approach him about stopping it.
I mostly agree with the previous poster that if you feel uncomfortable with it, then it may be a problem.