That's not at all how it works. Lots of companies--in fact, most--choose to cover married couples but not domestic partners. Domestic partner benefits are a recent innovation and on the east coast anyway they're usually limited to people of the same sex. If a company refused to cover spouses, they'd set themselves up at a competitive disadvantage, but if they did make that choice, it seems extremely convoluted to blame it on a previous expansion that would only cover a small minority of employees.
My company has tons of married individuals but very few people who I think might be claiming same-sex domestic partner benefits.
Also, it makes me uncomfortable to see this discussion resolve to "who can we lock out of the health care system."
But why should I (or my employer) have to either pay higher premiums or cut back coverage for others to pay for the expensive medical conditions homosexuals have. Insurance isn't about spreading costs equally; it's about allocating costs according to risks. Male homosexuals have particularly high risks.