I feel like everyone is missing the point.
If you have a child over as a guest, you should be respectful of their lifestyle and belief system. You wouldn't force a child not to pray before a meal because you don't for example, would you? While some of these things may not seem dangerous to you, that's beside the point! It's not your child and to tell someone you may not be able to fulfill their wishes is like saying, "I'm not gonna take care of your child like you do and tough crap!"
As several people said, you typically surround yourself with similarly minded parents and families and it's not a problem.
I agree that the child should be told to speak up for himself but you should also coach them and teach them to offer alternatives: "I'm not allowed to play that. Can we play legos? Can we go ride bikes?" The point of shielding your child from these things is to teach them about what is appropriate in general---I mean, it's not like you would be ok with your child being overly violent just because now he's 16 and not 6! I know studies were being done about the connection between violence/shooting at schools and violent video games (obviously not talking Mario Bros here!). I say to my children "we don't like you to watch that because it is not appropriate for children" or "we don't like you to watch that because it is really violent and we as a family don't watch really violent stuff."
Don't get me wrong, my DH and I LOVE movies and play video games on occasion ourselves and our kids have seen some PG and PG-13 movies under our supervision. But I would be totally respectful of other families who don't believe or watch as we do.
I guess I'm trying to say that what a family has as guidelines for movies and video games is about their values and their morals, which is very personal and should not be poo-poo'd or put down.
The other thing is, I am very surprised at how judgemental people are being! I don't like SpongeBob for my kids either! And it's not for you to tell me or anyone else how they should be parenting or that they are being too strict. That's really none of your business! Everyone has different parenting styles and should be willing to compromise. I would say, if a parent came to me and said "I don't want my kids watching My Little Pony" and I thought, "well that's crazy stupid!" I'd say, "Ok, no problem! Outside playdate it is!" or "Ok, we'll play playdoh!" And you just adapt to each other's idosyncracies. At least you know that when your child goes to their house, there will be absolutely nothing inappropriate seen or played by your child!
Alternately, if your child really likes another child but you know their household is not like yours, just always suggest they either play at your house so you have some control or tell your child they have to play outside, period, he is not allowed inside their house. That's it!
My son is told that violence isn't good and that gory things will give him nightmares and so he will tell people all the time, "I can't watch that! It'll give me nightmares!" lol
So I think it's just about being careful, communicative with both your kids and the other parents as well as teaching your children about good values and what values you have as a family.
But I also have to agree with someone, I'm not sure who said it, but if you are the step-parent, it's really your spouse who should be setting the guidelines and seeing them through in my opinion. I think that is Dr.Phil's opinion too! lol I'm just saying, maybe you shouldn't be so stressed about it, as this is really your spouse's job! Just a thought!
Good luck!!