Good point.
I guess my point is that I believe the Court may have been in error in not striking down Section 2.
Once Congress arrogates this power to itself, what is to keep it from deciding that custody decisions, divorces, etc. by one state need not be respected by others?
The point is moot, of course, since the present, and probably any future, Congress would not pass a DOMA. In fact, they are more likely to pass a law requiring every state to accept any marriage legal in another state. Which, if you accept the constitutionality of Section 2, seems well within their power.