Newbigin, Mission and the SSM Debate

One of the authors who never fails to stimulate my thinking is Lesslie Newbiggin, I'm currently reading The Open Secret: An Introduction to the Theology of Mission. Towards the conclusion of a long and nuanced discussion of the relation of (cross-cultural) ethical issues to Christian conversion Newbiggin states,

As a learning community that can only press forward from partial to fuller understanding of the Father's reign, the church will know that it cannot impose it's own ethical insights at any one time and place upon those whom the Spirit calls into its company. It must always press on toward fuller obedience but at the same time proclaim Christ as Lord and beyond its own faulty obedience, and expect and welcome the correction of those whom the Spirit calls into commitment to Christ. (p140, rev. ed. 1995)

His argument is in relation to cross-cultural mission and how missionaries (presumed western) impose ethical requirements upon new converts (presumed non-western). It is a convincing argument and I found no fault in his logic, but the only example he gives is polygamy. The story (possibly apocryphal) comes to mind of the African convert who is told by a missionary that as a Christian he cannot have more than one wife, so the faithful new Christian goes home and kills the four other wives that he is not allowed. A horrible result and if true reveals as much about the missionary's obsession with sex than the Bible's teaching against polygamy (try and find some).

My question, and I don't find an easy answer, is how this might relate to our western context in NZ where we have been increasingly (and rightly) taught to consider evangelism in western countries as cross-cultural. In a sexually permissive 21st C. western society is same sex marriage parallel to polygamy in 19th C. Africa? Is the gay community an unreached people group that needs to be allowed to establish it's own ethical convictions in relation to the gospel without the colonial imposition of standards from Christendom? Certainly that might provide a way out for the current Baptist Union discussion, to view the small number of churches accepting SSM to be frontier mission churches where the rules need to be looser. But that feels a bit difficult to swallow, after all even if we categorise the gay community as an unreached people group (I don't know if we should or how they might feel about that) it is one thing to permit ethical flexibility among marginal groups coming into faith it is another thing to have established churches preside over and facilitate the ceremonies that confirm people in that behaviour.

What do you think?