Even the churches seem to accept that a wedding may not be followed by children, and most seem to be very happy to marry couples who are well past the age at which conception is likely to occur.

Since it appears that conception is not actually essential to the concept of "marriage" but merely on the wish-list, then what is the fundamental characteristic that distinguishes "marriage" from an arrangement which is apparently identical in every way other than the gender of the participants?

If, as some people seem to be suggesting, that gender difference is of fundamental importance, could they please explain why, if conception is left out of the equation?