how do you support "the rich tapestry of human life and the desire amongst people to express their own identity in the manner they see fit" without prohibiting yourself from criticising abhorrent cultural customs like FGM?

One common approach is called "liberalism". It ascribes certain notional boundaries — called "rights" — to each individual; and asserts that each individual may do as they choose to express their identity, so long as they do not transgress the notional boundaries of another person. This places certain limits on the ways each person can "express their own identity in the manner they see fit" in order to define a space in which all others can do so too.