Quite a lot, it seems. Traditional marriage is between one adult man and one adult woman, ostensibly for life, and in most cases with the desire to beget and raise a family. In addition to traditional marriage there are many other types of relationships, including polygamy, communes, two women or two men in an exclusive relationship, menages de trois (or more), open relationships, or just co-habiting (sometimes called ‘common-law marriage’).
Gay marriage is now legal in many countries, but has caused a lot of controversy particularly from religious groups who regard marriage as being between a man and a woman. It seems to me that the terminology is what is upsetting people more than the fact that two people of the same gender are living together as sexual partners and soul-mates.
Personally I could see nothing wrong in civil partnerships when UK brought them in for LGBT people. My life-partner and I would have been overjoyed to have a civil partnership and put everything on a legal footing, and have our relationship (which lasted 21 years on Earth till he passed to Spirit in 1991) recognized. I never looked on him as my ‘husband’ or ‘wife’, and he never referred to me in that way either as far as I am aware. Again, terminology is important. He was my life-partner or spouse. I now feel I am his widower, and this term seems suitable for any man whose life-partner has passed on whether they were legally married or not, while a woman would be a widow.
I don’t think civil partnerships made LGBT people ‘second class citizens’. Two men or two women in a relationship are of equal status to a man and a woman who are together, but the relationship is different. For a start, provided both man and woman are fertile, there is the possibility to have children between them. This is not possible with two people of the same gender, though they can adopt, each can begat children with a partner of the opposite sex, or they can use surrogacy/IVF. Many in same-sex relationships have no interest in raising a family.
I really see no problem in having different terminology for different types of relationships, so long as all have equal status. If people want open relationships or polygamous ones, that’s fine by me, but don’t call them ‘marriage’. As for LGBT partnerships, calling them ‘marriages’ seems inappropriate to me. The alternative, of course, is to qualify the type of marriage, thus you could have a ‘traditional marriage’, a ‘gay marriage’, a ‘polygamous marriage’, an ‘open marriage’ (open relationships with two main partners).
Maybe I’m just old-fashioned, but it seems just using the word ‘marriage’ unqualified when it is not a traditional one between a man and a woman is bound to cause resentment.