[Avodah] Mezuza On The Home Of A Mixed Married Couple

[Sholom Simon]

>Someone offered a mezuza to a Jewish man living with a non-Jewish 
>woman.  The man agreed to pay for it and hang it up on his 
>door.  Rav Moshe Shternbuch was asked (3:327) if it was worthwhile 
>to help him at least keep this one mitzva, despite his grave situation.
>
>Rav Shternbuch answered that a Jew who lives with a non-Jew has a 
>Din of a Meshumad, someone who converted to another religion.  A 
>Meshumad is not even entitled to be buried in a Jewish cemetery, and 
>neither is someone who lives with a non-Jew.  This is worse than any 
>other aveira in the Torah.  He is not merely Oveid Avodah Zara he is 
>a Meshumad.

What makes it worse than any other aveira?  Why is it worse than, 
say, being mechallel shabbos b'farhesia?  Or, to take an example from 
Rambam, wearing sha'atnez every day?

-- Sholom