I an not even going to bring up homosexuality. But how do you define 'a husband' and 'a wife', given that some people are born with a varity of genitial birth defects. I am not even going to bring up the psychological gender birth defects, instead I am just going list one of the physical ones that could be used to kill the 'defining husband and wife' arguement.
'Androgen insensitivity syndrome' a person is born with XY chromosomes and testes, but has outward female genitial, though having not internal female reproductive organs. The person's cells are immune to testosterone, during puberty this cause a overproduction of testosterone which the body naturally converts to estrogen. Physically the person develops like an average girl in body measurements and usually has the mental view of being a girl.
Now how would you define this person, male and female? Or someone inbetween who you just deny the right/privilege (not sure which marriage is) to get married to anyone because of birth defects that they were born with?