I think it's to do with the way he views sacraments. I tends to view them as just a physical representation of something which is going on spiritually anyway, without much efficacy of their own other than their psychological effect. So if two people wished to be married, and felt that this was what G@d was calling them to, but couldn't do it the traditional way because they were stuck on a desert island or both of the same sex, they could become married by mutually agreeing to that commitment and doing whatever they felt was appropriate to mark it.

But I think that that was basically what usd to happen in the olden days anyway.