OK.. so homosexuals want their marriages recognized, just to get hold of the benefits. In that case, why can't we extend the benefits to unmarried homosexual couples? There will be no need to go through a sham marriage.
You want to go on a first date and have to pay alimony if you don't call them again?
This is an important point though. I think all of the legal benefits (and drawbacks) of marriage should be separate from any religious component. There should be a legal marriage available to anyone (straights, gays, atheists etc.) and then religious institutions can decide which marriages they want to bless and allow into their church/mosque/temple to do the marriage ritual.
For a lot of couples the religious component is important, but only the legal marriage should provide tax benefits or spousal privilege or all of that stuff. Religions can decide who gets to do their type of marriage, but under the law everyone should be equal.