Has marriage become a Federal issue since...?

Social Security, Railroad Retirement and the VA pay benefits to spouses. That's why the status of being legally "married" is important.

Social Security for example will pay spouse's benefits to same-sex spouse's if the state they live in recognizes same-sex marriages. Even though social security is a federal agency they follow state laws when determining relationship issues. Let's say that Utah recognizes same-sex marriages. If a spouse files for benefits in Utah he would be entitled; if they move to a state which doesn't recognize same-sex marriages he wouldn't be entitled under current law. This is unusual and I imagine this law will be changed to recognize a same-sex couple as married in all states if they were married in a state which recognizes the marriage as being legal.

Example - some states still recognize common-law marriages. If a couple has a valid common-law marriage recognized by the state then social security will recognize it as being a valid marriage as well and spouse's and widow/er benefits can be paid - regardless of what state they might move to. Ohio recognizes common-law marriages; Michigan doesn't. If someone enters into a legal common-law marriage in Ohio and they move to Michigan social security recognizes the fact that they have a legal marriage. I suspect the same-sex marriage will be treated the same way eventually.

If the federal government itself recognizes same sex marriages then the state issue is a moot point because ALL states would then have to recognize same sex marriages as being valid.

No.