marriage is a legally defined principle, determined by legislation. it is a set of laws that describe who can do what and what the advantages and benefits (and disadvantages) are.

there are (obviously!) lots of laws about lots of things. like who can drive a car, how fast and in what direction, what language you are allowed to use on television, who can vote, who can drink at the water fountain. these laws are set by legislators and can be changed by legislators. and they have over time been changed by legislators to reflect social changes.

the laws regulating marrige are no different. they can be changed by legislation and have been changed.

the marriage laws changed to allow marriage to be done by someone other than a priest. they changed to allow divorce. they changed in relation to age limits.

the simple answer is the laws regulating marriage can be changed and have been changed as society has changed. there is no reason why they cannot be changed again to reflect the more recent social changes, and recognition of homosexual relationships is one of these recent changes.

i would guess that decades ago there were conservatives arguing against the divorce laws saying "marriage is for life! if we allow divorce it devalues marriage! it is against our christian heritage! it is unnatural for couples to separate! it will lead to the disintegration of society!!"

sound familiar? but the sky did not fall then, and i cannot see why it would fall now.