It is completely legal. That's because opposite sex domestic partners have a long-standing,  readily available and well-known way to get the right to be on each other's health insurance (not to mention getting tax advantages, inheritance rights, etc.): marriage. The reason that in many states, health insurance is made available to same sex domestic partners is because, until very recently (and only in some states), gay marriage was not an option--they lacked the chance of marrying which any opposite sex domestic couple had. Since opposite sex couples can marry, they don't need to have the right to be on each other's health insurance without marrying.