Issues that tend to cross state lines, sometimes need Federal regulation. Marriage is of that nature, because existing benefit legislation forces national companies and insurance companies to set up processes for gay marriage the minute one state allows it. States have to decide how to treat gay marriages or civil unions when the people relocate to their state, etc.
If there is no need to federalize some stuff, then why do you think the founding fathers ever federalize anything?
No, you just need firm state laws and firm state supreme courts.
The convenience of a federal law or constitutional amendment is NOT a reason to have one.
No, it doesn't. Insurance companies have to deal with various state regulations for interstate companies all the time. Illinois requires fertility treatment. Aetna doesn't have to cover fertility treatment in Missouri because Illinois mandates it.