Perhaps the parents you're referring to are failing to realize just how strongly gendered their lives are.
But same sex parents report similar things happening. And many parents try really pretty to fight aspects of this - talking about movies and tv shows; not conforming to the roles; etc.
Yes, that's a large part of it. I think people also underestimate how much children pick up from society in general. Beyond what they get from their parents, they're still going to be meeting friends and family members, going outside and seeing other humans interact, other people are probably going to be buying them gifts (even if you try to avoid this, there will be times when people spring them on you), etc. There are a lot of influences we've stopped noticing because they're so pervasive.