If--big IF--the U.S. Constitution requires states to recognize out-of-state gay marriages, then no federal law can change that fact. In other words, you can't end-run the Constitution on Congress's say-so. That's not how things work in this country.
On the other hand, if the U.S. Constitution doesn't require states to recognize out-of-state gay marriages, then the law isn't even necessary, and repealing it would have no impact.
The courts will eventually have to interpret whether states have to recognize other states homosexual marriages. The Defense of Marriage Act says they don’t have to, but that has not been tested in court yet.
The other piece of the Defense of Marriage Act that nobody here has mentioned is that it defines marriage for all purposes of federal law as one man and one woman. So, even if states were forced to recognize Massachusetts and Connecticut same-sex marriages for purposes of their own state laws, the federal law of 1 man and 1 woman would be unchanged. That also has not been challenged in court, yet.