He may be joking, but it’s an intriguing shailah. You may have to look into the civil union/marriage laws first. In many cases sham marriages for legal benefits are criminal fraud. This happens often in immigration settings, where people may “marry” a U.S. for the sole purpose of getting fast-tracked for citizenship status. Immigration officials often do extensive investigations to determine whether it is a real marriage – did the couple date, did they invite family to the wedding, do they live together, do they know about each others’ backgrounds, ect.
So two yeshiva guys doing this may be fraud, aside from any other more complicatted halachic issues related to maaris ayin.