Considering that such an abuse of marriage is not a serious problem now (except perhaps in matters of immigration), it's rather unclear why it would become a problem in the future. After all, as you pointed out, everyone's allowed to get married as it is, including gay people, as long as it's to the opposite sex. So anyone who wants to get married for fraudulous purposes while carrying on with the partner of his or her choice, can do so as it is. And considering the relative rarity of homosexuality, and the amount of people who still have a problem with homosexuality, a fraudulous same-sex marriage would have the distinct disadvantage, compared to a fraudulous heterosexual marriage, of drawing attention. I'll give you points for originality, though, as I don't believe anyone's ever used the "but a mobster boss could marry his right-hand man so he can avoid having him witness against him!" argument to oppose same-sex marriage before...

And then there's the part where you mentioned "couples that will be deprived of the traditional benefits of marriage because the costs of extending them to same-sex "marriages" has forced them to be withdrawn". You do realize what kind of numbers we are talking about, yes? Gay people make up 5 to 10% of the population, and so far in the places where it's legal, gay couples are not getting married at a higher rate than straight couples or anything like that. As such, the increase in marriages and in people who are married and claim marriage benefits will be quite modest, certainly not of a kind to cause financial difficulties to those providing benefits to married couples.