Do not let yourself use the "to each his own" argument. Its PROVEN (like 2+2 = 4) that that outlook is plain ol' wrong (you can find it on wiki or something).
What about people who roam around naked? Whats wrong with that? Its their bodies, who are we to discourage them? What about child molesters, they are having CONSENSUAL sex with young teenagers, who are we to stop them its their bodies.
Please pick a better argument.
Now: Treating homosexuals as outcasts is no worse than treating black people as outcasts for the way they are born. These people are not doing anything wrong (some might even argue that they are doing right by our society by helping with overpopulation problems) and they just want to be treated as normal people who happen to do something you don't completely approve of, not as criminals, or outcasts!
In any case people have discriminated against groups for all of recorded history. So we need to stop it as much as possible because from ALL of history we can see what good comes out of discrimination.