My basic premise would be that sin is something which harms yourself or harms others. In Jesus' time pre-marital sex or 'fornication' would have been much more serious due to diseases, overpopulation, etc.

Alternitavely, sinning could be defined as pursuing your own self interests AT THE EXPENSE of the whole.

I do not really believe God has any set of rules other than love God and love your neighbour. If he really did have a set of rules for us then why doesn't he just implant them into our brains?

To sin is not something you should plan. My question is, if I go out and have sex with other guys in a NON ABUSIVE, CONSENSUAL and PREMEDITATED manner, then how am I sinning?