There's been much written on avoiding temptation to be unfaithful in one’s marriage, especially concerning practical steps that can be taken. The practical is, indeed, important, but more so is the tending of one’s inner life, where scenarios develop in the first place. These scenarios can cause us to lose mindfulness of the practical aspects. (I.e., we don’t care about minding natural boundaries when cultivation of an inappropriate scenario occupies our inner life.)
The first boundary crossed when seeds of unfaithfulness germinate is the view of another person as less than other, in all the aspects that the person is "other." What separates mere friends from potential adulterers is that there is always a sphere of the friend’s life that one will never enter, whether in thought or in deed. When one ceases to think of the friend as “other,” though – other in certain aspects of life, person, and marriage – one starts to imagine thoughts of ownership of that person that are untrue. This is a wholly different concept than that involving mere consent.
(Although the violation of the otherness of a person will, at some point, meet with legitimate consent issues involving someone...if not another person, then ultimately God.)
There will certainly be a common sphere shared by friends -- a sphere of common interests and experiences, as well as a bond of caring and fidelity. But part of that bond of caring and fidelity is to never violate the “other” of that person.
Contrast this with one’s spouse – a spouse must never be an “other!” And, not to be misunderstood, I don’t mean that he or she is not to be honored as a distinct and separate person. However, marriage means that two become one, and thus one's spouse is privy to one’s living space, possessions, and habits as well as one’s thoughts, wishes, troubles, everything – by the nature of being one’s spouse. Trouble happens when one spouse or the other denies this belonging and withholds or rejects what ought to be shared. (Sharing involves both the giving and the receiving) It’s a sure-fire intimacy-killer, and tills the soil for unfaithfulness. Faithfulness means being faithful in every way to everyone involved – faithful to both offering one’s “things” and taking in one's spouse’s shared things, as well as never giving to, or taking from, any other what rightfully belongs to a spouse.
This concept of "otherness" applies to many things, not just marital fidelity. Perhaps in our culture of ease and sense of entitlement, though, we've come to think that whatever we want (or think we want), we can have. Yet the ten commandments speak of coveting, and, though they address things such as neighbors' possessions, the issue is not limited to these things. Just about every sin imaginable starts with the prideful coveting of something that has not been granted us. We sin because we want what is not rightfully ours. But would that we'd all enjoy the bounty that the Lord does grant us, allowing that which we might desire wrongfully to always remain "other."
