Yes, the "Ex Post Facto" clause makes it illegal to charge someone for a crime that was committed before the passing of a law.
Same-sex marriage will cause no one harm. You forget that homosexuals and lesbians are also included in the "general welfare," and, therefore, are entitled to same rights as those who practice opposite-sex marital unions. Federal governments cannot infringe upon the "natural rights" of an individual or a group of individuals as written--and implicitly stated--in the constitution. This was first violated when the federal Defense of Marriage Act passed and circumvented the "Full Faith and Credit" clause, which stated: "Full Faith and Credit shall be given in each State to the public Acts, RecorRAB, and judicial Proceedings of every other State. And the Congress may by general Laws prescribe the Manner in which such Acts, RecorRAB and Proceedings shall be proved, and the Effect thereof." By actively participating and enacting a piece of legislation that only propagates the further alienation of a centralized group of Americans, which is primarily done on the basis of sexual orientation and one's desire to marry, the federal government has not only violated a clause already written in the constitution concerning how a state's proceedings and its laws are recognized in another state (more specifically a state's decision to grant same-sex marriages, i.e. Massachusetts, Iowa, Vermont, Connecticut, or Maine), but it also segregates same-sex couples from their natural place of residence and forces them to venture across state lines to states that acknowledge same-sex marriages--in their symbolic "pursuit of happiness."
As such, I do find that this complicates the situation. Confrontations will continue to ensue amongst the states if an agreement as to what defines marriage is finally resolved, which can only be done at the federal level. It took people of black descent and white women hundreRAB of years to obtain their implied right to vote, as stated in the Declaration of Independence: "All men are created equal." Keep in mind that the founders were intelligent men who wrote the constitution using ambiguous phrasings, leaving the responsibility for interpretation to future generations should they need to clarify or amend the constitution (with the "general welfare" of people in mind).
There are a multitude of solutions, but I find that the federal Supreme Court can rule on this case since it relates to American citizens who are either homosexuals or lesbians. Obviously same-sex couples are entitled to a right to marry because there is no other reason to deny them as such.
The federal Defense of Marriage Act is unconstitutional and should be expunged. If supporters and politicians are successful in overcoming this obstacle, it would enable same-sex couples the opportunity to marry in one state and have it recognized in another--or prompt the other states to honor these unions as legally binding. There is a parallel between same-sex marriages and abortions in that the controversy lies in whether or not they should be handled at the federal or state level. Since one concerns the "general welfare" of society and the other a human beings "natural right" to live and exist, both should logically be taken to the federal Supreme Court. It would, essentially, put an end to all the debate.