If it's in the Bible it's not a sin?

I was listening to a Rich Mullin song (Jacob and 2 Women on his album The World as Best as I Remember It) the other day and was struck by these lyrics:

Jacob he loved Rachel and Rachel she loved him
And Leah was just there for dramatic effect
Well it’s right there in the Bible so it must be a sin...’

The reasoning here is:

P1: Jacob having two wives is in the Bible.
[P2]: Anything recorded in the Bible is not a sin.
T1: Therefore, Jacob having two wives is not a sin.

The problem with this argument is P2 (which is the implied assumption required to lead to the conclusion). There are many things recorded in the Bible which are clearly sins, eg, murder, rape. So the fact that something is recorded in the Bible is never enough to conclude that it is or is not a sin. If it was, then we could engage in all sorts of behaviours which are clearly proscribed elsewhere in the Bible!