and of those who do, the actual "marriage" or civil union lasts on average under 1.5 years.....much less than the uptake rate or average length of hetero marriage.

Where are the figures to support this contention? If there are any it must be taken into account that same sex marriage was only legally recognised by most countries at best within the last decade or so. There are still many places that do not recognise or allow these unions and so reliable figures won't be available. How can reliable statistics have been compiled in so short a time? There will be no records to show the number of same sex couples who have been living in 'sin' for longer periods in long lasting happy relationships.

I agree with Neil.D. If the spread of HIV/Aids is a reason for discouraging same sex marriage for men then same sex marriage for women should be encouraged. Makes sense to me, if that is the premise from which this situation is viewed. However, I see abuses of marriage in heterosexual unions - Kim Kardashian comes to mind as the latest of a long line of stupidities - which make a mockery of the marriage vows.