You need to read the bible more. In Matt 16:19 Jesus is talking to Peter when he says this. However in Matthew 18:18 he is talking to ALL belivers. There is no special 'papal' authority in the bible. God is not a respecter of persons.
Some Christians marry, divorce and remarry. That isn't allowed by the Bible but it's still done. It's still not allowed by Catholicism.
It's not allowed by protestantism either, except when the divorcing member has a valid biblical claim against their spouse (that is, the spouse committed adultery).
However people still do it. Just like Nancy Pelosi and other Catholics support abortion and all sorts of other evils.
Oh, okay, whatever you say.