Your probably thinking about the good ole days. "Two women are together" Lets put them on stakes light them on fire and have man on women relations while taking in the sweet smell of their flesh burning and their screams echoing through our relations"1. Think of it like this. God created a system where breeding occurs through a man and a woman having ***. And God wants us to "Be fruitful and multiply". So you can see why he wouldn't want us be messing all that up.
2. Because sure you may be kind and help out the needy and never hurt anyone or steal anyone's car but you won't be sin free. At many points in your life you may steal someone's pencil or tell your fair share of lies, look upon someone with lust, have bad thoughts, maybe you even say mean things now and then. Now if you did none of those things then I'm sure you'd be all peachy keen but who was the last person to lead a sin-free life? Oh yeah, Jesus.
3. Well, read above. Even if it is just a minor sin. Wrong is wrong.
Why do you religionists ignore that? It's like you think that is wrong.