Not telling is lying

TL;DR: I don't have a clear guideline for when lying is right or wrong, but I have one against ontological obfuscation.The Behavioral Intentional Stance (BIS)A man meets a woman. In the following days, they meet repeatedly, grow physically closer, eventually have sex, and after a few more days have a conversation in which the man tells the woman that he has been seeing other women throughout this process. He didn't bring it up before because he was unsure of how she would react. Has the man lied?Language draws arbitrary lines in Thingspace, so two different definitions of lying could easily place the man's behavior on different sides of the line, but the question here is whether a natural joint to carve at can be found.There is one: the intentional stance, or rather an adjacent concept I'll call the "behavioral intentional stance". One's beliefs about the world adopt the (epistemic) intentional stance towards X when they model X as having preferences and acting based on those preferences.[1] It's obvious that a liar still adopts the intentional stance epistemically when dealing with the target of their lie: they are aware that the target disprefers being deceived, and so they conceal their deception. But something adjacent to the intentional stance can be adopted or not adopted in one's behavior.Every decision is an optimization problem: there is a set of preferences, and acting means finding world states that optimize for those preferences. To behaviorally adopt the intentional stance towards X means including X's preference in the set being optimized over. Preference sets are often incompatible, so adopting BIS towards X often means recognizing that the naive picture of how a collaboration might look is actually unworkable. Sometimes a slightly modified, compatible collaboration plans is easily found; sometimes it isn't.[2]Main claim: There is no middle ground between adopting and not adopting BIS. One can of course adopt it partially, i.e. adopt it towards collab