Montague had contacted the school 'and asked for her son to be excused attendance from the parade.
A devout born-again Christian, Izzy Montagu, 38, is suing her four-year-old son’s school for compelling her son to take part in an LGBT pride parade.
LGBT is an initialism that stands for lesbian, gay, bisexual, and transgender.
Montagu said she was told by the headteacher of Heavers Farm Primary School in South Norwood, southeast London, United Kingdom that her son could not opt out of the event in June 2018, Daily Mail reports.
The school sent a letter to parents on 19 June inviting them to partake in a Pride march and 'celebrate the differences that make them and their family special.'
A week later Montague contacted the school 'and asked for her son to be excused attendance on 29 June as they were concerned of him being involved of a public display of adherence to views which she did not accept.'
Montague's request was refused by the headteacher Susan Papas so the parent replied with a lengthy email on 13 July, Central London County Court heard.
At the start of the eight-day case Judge Christopher Lethem described Montague and her husband as 'devout born-again Christians, and they bear a belief that sexual relations should be abstained from or take place within a life-long marriage between a man and a woman and any activity outside those confines is sinful.
'They also say pride is considered to be the most serious of the deadly sins.'
Montague also told the court that she had expressed concerns to her son’s teacher about the types of books they were reading in class.
Montague said: 'There was one reference to a same-sex family book.'
Representative of the school Ian Clarke said the book is called the Family Book
Montague, supported by the Christian Legal Centre, is suing the school on the grounds of direct and indirect discrimination, victimisation and breach of statutory duty under the Education Act 1996 and the Human Rights Act 1998.
It is the first time that a UK court will scrutinise the legality of imposing LGBT ideology on primary schools.