I would like to argue that any type of social contract and commitment of the form which appears in marriage, those that involve interactions of individuals’ personal lives are fundamentally unethical.
Given the subjectivity of value systems one particular action can be thought both justified and unjustifiable, in the same time, but with respect to different perspectives. (From now on, I shall refer to perspective as reference frame for the reasons of personal interest.) Nevertheless, marriage is an interaction which causes the personal aspect of people’s lives not to remain separable. On the other hand, while this entanglement is formed, marriage doesn’t give any mechanism to create a shared a reference frame, which is a source of catastrophe in these type of commitments, i.e. there is no way of having the same value system for two different people in all important aspects of their lives.
As a trivial idealised example consider a heavy smoker who knows all the consequences, but believes smoking is a vital action for him/her to maintain a happy life. So he/she chooses to do so… There is nothing here to argue, as he/she has chosen to perform an action consistent within his/her reference frame. Now imagine he/she loves someone who really hates smoking. In such a case at least one person should give up her/his attachment to the a priori value system.
What is more subtle is even given a shared reference frame, lack of a common threshold will make the relationship again unethical. As one may like something up to a threshold, not being bothered by it up to a second threshold, but as soon as that thing gets more, it’ll become intolerable for him/her. While the same intolerable could be pleasant for his/her partner!
More dramatically, by the entropy arguments, one can say the more socially sensitive the individuals are (the more stuff they have to care about) the deeper this conflict gets.
One might argue that by this reasoning almost all of the human activities are unethical, as most aspects of their lives are in one way or another entangled. Nevertheless, this is irrelevant. Because most of the events in people’s lives had not much influenced at any time when their backward light cone had overlap. Of course those which did have any noticeable effect should be carefully considered, but there are surely very few of them.
One may still choose to act unethically for the benefits she/he gets from these accepted norms of the society. At the end of the day it’s the society which finds its ways.
One question remains: whether it is possible to arrive at a conflict free personal interaction, in the sense discussed above. I think the answer is not obvious. But I believe the least price would be departure from a minimal set of today’s accepted values, e.g. monogamy or accepted commitments in partnership (e.g. non-open relationships).