Why do you think that Mr. Blanchard suggests that Rick and Gillian wait for six months before they marry?To Sir, with Love by E. R. Braithwaite

After Rick's introduction to Mr. and Mrs. Blanchard,
being aware of the deep prejudice against mixed marriages at the time of this
narrative, Mr. Blanchard broaches the question of what Rick and Gillian will do if they
have children since often mixed racial children are not accepted by either race.  Rick
replies rather acridly to this question, but Mr. Blanchard ameliorates the
situation, saying that he just asked because he knows that they will encounter some
social prejudices. 

Then, Mr. Blanchard underscores his
understanding of the difficult step that his daughter and Braithwaite are taking by
suggesting that they wait about six months before they marry. By doing so, they will
have time to become more knowledgeable of each other and to meet people as a couple and
realize what prejudices and difficulties they may
encounter. 

It is worthy of note to mention that this novel
was published in England in 1959, before the Civil Rights Movement in the United States
and before Great Britain accepted African Majority Rule in Africa where Britain had
colonies.  The British were strongly against interracial marriage at this time. 
Therefore, it was uncommon for couples to be mixed races.