if they want children, they adopt children or have a surrogate.
why do you care?
and do you believe that older couples who are no longer capable of having kids shouldn't marry?
or people who are infertile?
Marriage has nothing to do with it.
I am talking biology
If you are naturally attracted to the same sex then your natural sexual orientation excludes reproduction.
People can be infertile but still be attracted to the opposite sex so their sexual orientation does not exclude reproduction even if they themselves cannot reproduce.
If it is your nature to be attracted to the opposite sex then seeking to reproduce is contrary to that nature.