It wouldn’t make it legal.Current laws outside of DOMA are by state by state and is up to inter state recognition(ie one state could theoretically not recognize your marriage). We leave it up to individual states to determine who can get married based on age, relationship..whether or not you have an STD..nature of the marriage(is it sanctioned, common law, etc)..so by repealing DOMA you are simply saying that the federal government accepts a marriage as defined by that indivdual state.
Right now all states recognize each other’s marriages, even common law, this temporary issue with homosexual marriage will be resolved in the same way, as you know.
Five or 15 years from now there will be no states that do not recognize what will become a common marriage practice between consenting adults, by that time polygamy will be legal as well, as we discard the definition of “marriage” totally, and use the word for all consenting adult relationships of that nature.