Whether it was wrong is largely up to you and your thoughts about it; you each (probably) have responsibility for the choices you make, you for having sex with your friend's dad and he for having sex with his son's friend and (presumably) outside of his relationship. I add "presumably" because we don't know the terms of his relationship, and therefore will likely project an assumed sexually exlusive relationship onto them.
I don't think you were wrong or right in the matter, nor that it was necessarily a poor choice. The primary thing for you is how you feel about your choice and that you can either stand by it or live with and learn from it. You should consider it with relation to your values, both the ones you hold and the ones you want to hold (if they are different). As they may be different from the values of other persons, you may come into conflict with others (such as in some of the replies to this thread).
Saturday, July 10, 2010
Forum comment - sexual wrongness
A fellow started a forum thread where he asked readers to evaluate whether he had been wrong in having sex with his friend's heterosexually-married dad. This was my input: