In To Kill a Mockingbird, when Mr. Gilmer hears Tom's remark he grows furious. What if such attitude became common?

First of all, to address your question, you must
understand that in this day, Negroes were thought of as lesser. Relationships,
particularly romantic ones, were a taboo of society. When Tom suggests that Mayella was
the one who made an advance toward him, he immediately (although respectfully and
truthfully) degrades her in front of the entire town. Gilmer can't believe that Tom
would do such a thing, it was not his place to get to say such a thing. But this was a
court of law and Tom should be able to tell the truth on the witness
stand.

This was a common attitude in the 30s. Today, in the
American society, romantic relationships see very little bias according to
race.