The Collins New English Dictionary describes marriage as " the legal union of husband and wife, -----, the religious or civil ceremony by which two people of the opposite sex become man and wife". Let us keep the word marriage as meaning just that. Should folks of whatever proclivities wish to live together then perhaps the term partnership would be more appropriate. They can quite easily enter into any binding agreements regarding their incomes, pension, properties, etc. as may any other individuals.
Regarding the raising of other peoples children or orphans, this presents a completely different set of questions and much consideration should be given before giving answers.