I will assume you are simply naive rather than assuming you argue for homosexual sex premised couplings benefits.
Marriage has been recognized by society and granted reward respect and privilege because of one thing -CHILDREN!
Nothing to do with love, companionship, sharing house chores, poetry, yadda yadda.
Society has never IMPOSED child bearing nor demanded proof of ability or intent. Any exceptions to the rule are for the most part ignored because they would be near impossible to glean and as such enforce.
FACT: Homosexual sex procreation is not an exception; it is an impossibility.
(WHY DO I FEEL I ARGUE WITH LEFTISTS ON FR, IS THE COUNTRY THAT FAR GONE?(
Your basic contention is flawed. Marriage has often been based solely on economic realities, e.g. inheritance, acquisition or exchange of property, social mobility, etc. That has certainly been the case until relatively recently in western societies and in many places it is still the case. My original point however, is that there may be no reason to assume that homosexual couples would be more likely than heterosexual couples to wed only in order to obtain benefits.