I will give you the explanation of my Church.
God did not create anything innately unclean in any spiritual sense, to think so is blasphemy. But the distinction of unclean in context is important: "holy" literally means "set apart". Just as the Jews are set apart for holiness, certain animals are set apart. But when the Jews rejected Christ (which according to Paul is a good thing for gentiles), it meant the covenant was now open to all peoples; in reflection of this, all animals likewise became clean. All peoples are open to holiness. And Peter's dream in Acts 10 is about this: the ancient way of baptizing is threefold immersion (this is still how my Church does it), and that is what the unclean animals being lowered with the sheet over and over represents. Baptism is what makes you clean now, not being Jewish, and God in showing all the animals being lowered three times shows they are all clean. To follow the old dietary restrictions, is the same as saying circumcision is what sanctifies you, not baptism. It's rejecting the now universal religion that God has given us, a religion for everyone.
Now you can follow any diet you want for all sorts of reasons, but don't go claiming God cares about a kosher diet. Now that the faith of God is a universal faith, what matters is your heart, and that is where God will look to see if you are unclean.