Can we require proof of a same-sex marriage?

Q. We received our first request for a same-sex spouse to be covered under our insurance. We’re fine doing this, as long as they are actually married. (We don’t cover domestic partners.) Can we ask for proof of marriage even if we don’t ask it from opposite-sex spouses? I’d like to create a policy that says if your last name is different than your spouse you wish to cover, we need proof of marriage. Is that legal?