The three personal reasons to legalise gay marriage are given in the reasons for marriage set out in the old Anglican marriage service. They also affect the whole society: The mutual comfort, help and support one for the other, with lifelong security; the rearing of children; and to give a socially approved and safe outlet for sexual desire.
The first reason is obvious for gay as well as straight couples. Most of us need a back up in our way through life – someone we can depend upon in sickness and in health, for richer, for poorer, and who will share our dearest thoughts and pleasures.