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.