Why is Hell Eternal When Sin is Temporary?

Sin is something that happens once and passes.  So the whole business of hell being eternal seems like it is WAY out of proportion.  Even in our human justice system, there is this idea of proportionality of punishment.  Doesn't it seem unjust that God would punish us eternally for something that we did during the short years of our earthly lives?

Free Will and God's Foreknowledge

If God knows the future, then doesn't that mean that I don't have free will? 

Why is God a "He"?

God is Spirit and does not have gender.  So why is God called a "He"?  I am hearing many accusations in classes that Christianity is sexist because God is labeled as a "He".

Denominations?

Why are there so many divisions and denominations when we all basically believe the same thing (the gospel)?  Isn't that a tragic thing that we have to be so divisive?  Why can't we all just be one denomiation?