Large corporations that have offices in several states and countries would like to be able to have the same health and benefit programs for all of their employees regardless of where they live. They would also like to be able to offer similar packages to employees who are required to transfer from one place to another.
It is easier and cheaper for them if all the states, or all the world for that matter, had the same rules and definitions regarding marriage.
That is why large corporations are in favor of globalization in general and legal issues in particular.
Of course some conservatives have been pulled onto the fascist bandwagon by rallying for insurance companies to be able to more easily sell their products across the country. This is supposedly some sort of expression in favor of free market capitalism. Large corporations don't care about free market capitalism, but they do like the fact that insurance companies can create huge nationwide monopolies, while other large corporations can simplify their benefit administration by putting all of their employees across the country under the same crappy and expensive plans provided by a single company.
Every state's laws are different. Same with mandated timeoff. I worked in HR software for awhile. Every year, the code had to be tweaked to permit this or that option depending on the state of the participants working for the client company.
Some states require time off exceptions (even if they are unpaid) for visits to vet, etc.
Every state's laws are different on a whole lot of things. Some states (like CA and WI) had 20 pages of such regulated exceptions. Liberalism is a mental disorder.