it's more than that, Quakers & several other valid recognised faiths want the right to bless same-sex unions; to vote against this bill was to deprive them of what they saw as valid religious practice. This for me is the crux of the matter, away from equality for specific groups.
Had a nice letter back from Norman Lamb about this :).
If an atheist woman and a divorced Muslim man turn up demanding to be wed by an orthodox Jewish Rabbi, the Rabbi won't be obliged, he'll still have discretion. Nothing is being forced on anyone (except under the traditional view of marriage system where Quakers etc. weren't allowed to practice their faith as they would like).
then why ban incestuous marriage or multiple partners in a marriage?
I hate the comparison with incest; Incest is obviously yucky, gay sex (or gay romantic love, anyway) is not yucky, or no more so than hetero sex/love.
Tax shelter situation gets too complicated with multiple partners, the whole picture of traditional marriage rights gets complicated, though it's not impossible (see Muslim traditions), but it's jolly tricky. That's why we mostly see multiple partners as a bit yucky, too.