Why does Mr. Raymond pretend he is drunk to help people cope with his mixed marriage?

I believe that Mr. Raymond does this because he wants to
live by his own values (he does not want to give in to what society says) but, at the
same time, he does care to some extent about other people's feelings and
opinions.

If Mr. Raymond truly did not care, he could just
say "hell with 'em."  He could be more in your face (as we say today) about his life and
not care about what others thought.  But he does care some about the other people's
feelings.  So he tries to give them a way to rationalize his behavior.  He tries to let
them feel that he only goes against society's values because he is
drunk.

So Mr. Raymond is not a complete rebel.  He does act
in a very rebellious way, but he still cares about the feelings of the people in the
community.

You can see evidence of this early in Chapter
20.