Hi Mulan:
I think I might have a reason for why the sexual sins are tolerated and the others are not.
For a JW to celebrate Christmas is to reject the authority of the JWs and their elders. One could say about sexual sin that they just are having a hard time fighting against those desires of the flesh which are so powerful. But, to say that the JWs are wrong on some point of doctrine is to reject them as God's official spokesperson.
A few years ago, when I was leaving the JWs my JW father made some interesting comments to me. He told me that he knew that I did not do the things that my younger brothers did. (Such has have sex outside of marriage, experiment with drugs, etc.) But then he said that I was worse off than my brothers because I reject the JWs as having "the truth". He said my brothers, even though they commit these immoral acts, at least still believe that the JWs have "the truth". (At least that is what he thinks, in reality they do not believe this but they will not ever tell him that to his face like I did.)
In any case, the worst sin a JW can commit in the eyes of another JW is not fornication, or stealing, or even murder. The worst sin a JW can commit in the eyes of another JW is to reject the teachings of the Watchtower Society. So, by celebrating Christmas, which is considered a religious act by the JWs, his son would be guilty of a sin worse than fornication. It is the sin of apostasy in his eyes.
Jeff S.