Try this and let me know what you think as an explanation
http://lesfeldick.org/lesqa-c.html#6c
See, just in this thread there are several explanations of what that sin entails... Each is plausible and you can make a good argument for it, but the fact is that there is no definitive answer in the Bible. So I choose to approach every person with the belief that there is nothing they have done that God cannot and will not forgive, if they will only ask Him and accept His sacrifice as payment for their sins.