Because marriage was originally/is a religious rite practiced in Judaism, Christianity, and Islam. The religious rite requires a heterosexual couple, as all three religions consider (in most sects) homosexuality a sin. The push for same-sex marriage comes primarily for a desire for the homosexual partnership to have the same LEGAL rights as a heterosexual partnership, but because of the use of the term 'same-sex marriage' the followers of said religions fight to prevent legal sanctioning of the partnership, in many cases only to protect sanctity of their ritual, and not because they really think that it should be illegal for homosexual relationships to exist, as the greater part of the population is rather tolerant of homosexual partnerships.
I don't think you can. You either has to be single or married to adopt. Just living together is neither. if you are in a samesex relationship and has gone through partnership might be possible though.
Anyone can do anything they want to as long as they truly want it. No, in Islam having a boyfriend/girlfriend type of relationship is not allowed whether your a male or a female and whether the bf/gf is Muslim or not. If you have anymore questions regarding the matter, just ask and I'll be happy to answer them. i gree in islam having a boyfrnd or a grlfrnd is not allowd but today no youngr generation follows it .....so if a muslim girl personlly like a non muslim boy...then it is not allowd in islam.....u cannot marry him unless he accpts islam...:)