The Consitution guarantees freedom of religion (religous plurality) not freedom from religion (atheism as the defacto national religion.)
Morality is not relative. Basic human characteristics, no matter the 'progressiveness' of politics, or modern advances in science, medicine, technology, comforts and conveniences, have not changed. All men are created equal (and in the same way, and made of the same stuff) and all are born both with a conscience, and with instincts and animalistic tendencies. It's what we do with those, with discipline, restraint, and judgment that separate us from the animal kingdom, and are a gauge of how advanced we are. Morality is the dividing line, or the barrier. You would have those barriers moved, shifted, pulled back down the hill, by your assertion that morality is relative.
The only problem is, you don't have the power to do that. You cannot change morality; all you can do is break yourself before it. One does not truly break the law. They transgress against it, breaking and demeaning their own character in the process.
Your concept of liberty is corrupted. Your ideas of freedom are perverse. True liberty is being free from worry, doubt, and restraint. If you are committed to living a moral life, the questions of whether you do this or dont do that are already decided. You don't have to hem and haw, and you are not weighed down by guilt, sorrow, and physical and mental addictions or habits. Of course, we all make mistakes, choosing to indulge here or there, at one time or another. That does not make it right. It does not absolve us from the consequences, which can be public, humiliating, putting us in dire straits in relation to family, career, public opinion, or can be very private, but nontheless damaging, and putting happiness and contentment, confidence in our standing in question, and putting us behind or below where we were previously. Those are non-changing truths. Momentary satisfaction from indulging in base behaviors has never ultimately lead to happiness. You can say it as much as you like, with as many as you can gather together. While legal laws may change due to the numbers choosing one way, those laws have no effect on morality.
The facts that you offer, of millenia of years in which adultery and homosexuality occurred, has not changed that it is wrong. Nor has acceptance by vast numbers of people of such behavior changed the real facts that people are indeed harmed. It might not be visible. The real hurt and damage rarely is. Because it is not there to be seen, now, does not negate its reality. Your perspective and attitudes are immature, foolish, self-indulgent, and dishonest. Your statement that same-sex marriage is "only positive" is crass, and merely an opinion.
Morality is never obsolete. Liberty and real freedom is something you have a long way back to discover. Your ideals about man and happiness and redemptive pursuits are sorely misguided.