The debate on gay marriage is making me uncomfortable. Columnists tell me breezily that I should welcome it because of equality. Catholic and Anglican Bishops say I should shun it because it undermines one of the fundamental building blocks of society. Now my instincts are liberal. I think the introduction of civil partnerships was a great achievement and I don't like the tone of recent church pronouncements.But there is an issue here which is in danger of being trivialised on both sides. The issue is the nature of marriage. Is it a convention or a sacrament? If it is a convention of course it can be redefined. But if a sacrament this is not so easy. The point about sacraments is that they can't be made up. They work because they are a precise configuration of the material and the spiritual. When I celebrate Holy Communion I need bread and wine. Tea and biscuits won't do because they don't carry the memory of what this sacrament points to, which is the bread and wine that Jesus declared to be his body and blood. If marriage is a sacrament it needs male and female because the inner memory of marriage goes back to the Garden of Eden - it is about procreative sex, the possibility of children, born from the bodies of both parents. It is all very earthy and concrete and physical.