"If you ask what is the single most important key to longevity, I would have to say it is avoiding worry, stress and tension. And if you didn't ask me, I'd still have to say it."
-George BurnsWell, fine!
The direst an argument should ever get between two lovers is "We'll just have to agree to disagree". A point should never be pressed to the extent that it presses into your lover's side; if both parties have exhausted all unpresented evidence, the debate is to be called to stalemate. Anything more will do nothing but build up enmity.
In the event of such a stalemate, it is often a good idea to, shortly after, do something small, nice, and unnecessary for your partner; the kind of thing where they say "Aww, you didn't have to do that". Remember how much you love them and let this bring an earnest smile to your face. This shows that despite the result of the debate, you are still on amicable terms; and is a clean solution to the period of awkwardness that usually follows these kinds of disputes.