I am abit concerned over a few points that have been raised. To throw in my 2cents adultery of the heart is the same as real adultery but does not make it ok to commit real adultery.
In the world of today it is illegal to attempt to commit terrorist acts, if I make a plan to blow up my school, or even write about it my diary or make a fictional story about it, the police will want to talk to me and i might be charged with something. This does then not make it ok to actually blow up the school.
As for the issue of "jesus forgave our sins anyway so why not do them" its not intended as a malicious argument. If i robbed a bank and shot a teller, then God my judge and jesus my lawyer decided to pull some strings and i got let off, would i then walk out and go shoot another teller? no.
But the issue isnt as straight forward as that. Humans are inherently flawed. God understands this and the church understands this. It is impossible to ever be free of sin because of who we are.
Why then is it so wrong to give in to our instincts? it is understandable to, and we are merely human. God has never distinguished between sins either. Sin is sin, just because everyone lies at some point in their life does not make lieing a worse sin than murder.
Wearing two blends of material in one peice of clothing is a sin, just as stealing is.
I beleive that if we live our lives according to our own moral compass, beleive in god and are not bad people than God would be more than happy.
And no, i have no idea why sex before marriage is so wrong. The only thing i can come accross is that God intended sex to be between two people only, a static couple. This does not explain why sex between two people who do not sleep with anyone else and then get married is so wrong.
Then again, god didnt intend for adam to get his arse booted out of the garden either.