If you live in a state that recognizes same-sex marriage, it will depend on whether your insurance plan is governed by your state’s insurance regs, those of the federal government, or those of some other state. If you are a small employer, it is likely governed by your state’s insurance regulations.
If your state recognizes same-sex marriage, and your insurance plan is regulated by that state’s insurance regulations, then in all likelihood “a spouse is a spouse” as far as the law is concerned, and I’m not sure you’d have any non-Constitutional recourse. I know this is true in Massachusetts, not sure about other states.
There are Constitutional arguments based on freedom to contract, freedom to associate, etc... but I’d consider them long-shots.
Of course, you’re always free to not offer spousal benefits at all...
Or not hire homosexuals at all, as someone else said. (In some states this may be illegal discrimination, but in most states it isn’t.)