FROGGEman2 said:
OK, this has left me feeling very patronised and emasculated, and enough people are just saying "You're a pussy, get over it" that I think I'll change the OP here.
Here's the new question: "What is the worst thing you have caught a parent doing?"
You were also given plenty of genuine advice and assistance that you apparently ignored or dismissed. The problem with your situation is there's really nothing you can do besides get over it or talk it out with the guy like two adults would.
Going behind his back to your mom with this will probably either make you look dumb (to your mom for thinking this was a big deal) or petty/untrustworthy/hypocritical (to the step dad for ratting him out, especially considering how the commonality of what he was doing). Best case scenario your mom will agree with you and get in a fight with the step dad over this little thing, which will make both of them (and probably you as well) unhappy. There just aren't many options.
If you have a problem with what he was doing and don't know why you feel this way, either tell him this or talk to somebody else about it. Maybe track down somebody who's like a mentor to you and talk about it with them. Really, there's not much more anyone can say or do for you.
If you're wondering why everyone is saying it's not a big deal and you should get over it, maybe it's because compared to a lot of family drama, it really isn't such a huge deal. Or perhaps they understand that it's unfair to expect parents to behave the way we think they should, rather than how we ourselves (and most other people) behave. So if I look at questionable materials, I shouldn't hold it against my father or mother for doing the same thing.
It's not that nobody wants to help, it's just that I don't see what anyone can really say or do for you.