People tend to live longer and be healthier when married. Why limit who can benefit from that?
Marriage has already expanded well beyond what it may have stereotypically been in the minds of some. For example, some people marry five or more times. Some marry persons of a different ethnicity. Some marry someone that is 50 years older or younger than themselves.
Instead of thinking of marriage as only sexual or for the purpose of creating children, why not consider marriage as a legal contract between two people for the purpose of support, kinship, love, companionship, respect, and commitment without regard to race, gender, or faith? How would that be perverted?