>"there is no reason whatsoever that the law should permit e.g. visitation rights to a couple composed of a male and a female, but deny them to a couple composed of two males or two females" //
Here's your estoppel right back at you - what legal reason is there then to deny that right simply because the grouping is not a couple but a threesome or because the couple are close family? If it were about equality in this way wouldn't you expect the outcome to be even handed??
As it stands the reason states have historically supported marriages with better benefits and tax breaks and such is that heterosexual marriage provides a strong basis for procreation that supports the long term upbringing of children. So there is at least one reason, which sure you can pick some holes in (that can be countered) but is nonetheless "a reason" contrary to your assertion.
>"If certain individuals of a religious persuasion wish to consider homosexuality a sin, fine. They don't have to practice it themselves, or even associate with those who do." //
The mirror argument to this is if those who have homosexual sexual relationships want to marry they may, they just need to find an opposite sex partner, fine. They don't have to get married though, or even associate with those who do.
There is plenty in the Bible that contradicts calling same-sex couples "married". Redefining the word "marriage" by statute was not in any way required in order to provide "equal rights under the law", civil partnership accomplishes this without interfering with religious ideals. The ruling was about religion.