Marriage certainly does approve of sex and procreation of offspring and it always should. Same-sex couples, like siblings, should not be approved to conceive offspring, it would be unethical and should be prohibited in each case. Donor gametes should be prohibited for everyone, it is not a right of marriage.
I'm not ahead of science, people that claim a right to procreate with the same sex are ahead of science, and it would be unethical and should not be allowed. It is unnecessary, and there is no right to, and it would not be "fun" for the subject of the experiment. People should not be created as experiments for "fun"!