Thurlow has long been a vocal opponent of mixed voice treble lines in cathedral choirs. In 1997 he offered his resignation to the Dean and Chapter of Chichester Cathedral after Salisbury Cathedral insisted that their newly founded girls choir should sing together with the boy choristers at the 1998 Southern Cathedrals Festival that was to be held in Chichester. Thurlow told a meeting of the Festival's trustees that his position would be made untenable if he were to accept this request, as he had "made such a strong case against this for so long" and "felt in (his) heart that it was wrong". Thurlow was backed by his choir, members of the Chichester congregation and the Dean of Chichester John Treadgold. Treadgold refused his resignation and a compromise was reached with Salisbury. Since 2013 he has been Vice President of Campaign for the Traditional Cathedral Choir, the controversial advocacy group that was established in 1996 as a direct response to the rise of girls' choirs in the United Kingdom.
