Of course it is a matter of cultural values. You have one set of values. An indigenous community has another set of cultural values. There is no reason that you are right and they are wrong no matter how strongly you feel.
I will admit that as a Western man, FGM is the most difficult example to accept. My belief in multiculturalism and anti-colonialism bring me here....and sure, I accept the uncomfortable consequence of my position
I believe the position that you are in.. Casting White people as the saviors of indigenous women, is at least as difficult. The idea that the brutality of colonialism was at all beneficial is an untenable (and racist) position.
I believe strongly that indigenous cultures were fine on there own without White people to "civilize" them.
I disagree with you about indigenous marriage practices. I agree with you that relationships between adults and children are harmful in a Western cultural context.
In other cultures the Western ideas of adulthood, childhood and marriage simply don't apply. It seems likely that if you lived in these cultures and had a chance to see that these marriages provided stability, family and met the needs of the community you might change your mind.
Even if you are right on these issues in some absolute universal sense, I still think the legacy of White Supremacy is too damaging to justify White people interfering in indigenous cultures.