The claim, that "Chances are you are both happier together as things are now," seems to me to be an empirical issue, not a philosophical one. Further, I can imagine many scenarios in which it is false. Much depends on what is meant by "happier." But whether "happier" means "the relationship as whole will be better/happier" or, instead, "your sexual experiences together will be better/happier," it would seem not to be a question that philosophical expertise could answer. I hope that our answers to sex questions do not come to mimic the pop psychology and psychobabble we hear sufficiently on the Jerry Springer and similar shows. Finally, the heart and soul of philosophy is argument, providing reasons for claims, including claims about morality and duties. In the answer to the question above, I cannot find a shred of argument. We should also avoid, that is, pastoral or friendly counseling. Without rigor, philosophy is nothing.

My response was composed for Steve B. But, with thanks for Jyl's intervention, let me try to rephrase in a way that Alan may find more congenial.

Suppose you were indeed morally obligated to tell your sex partner when you fantasize about someone else whilst making love to her. This obligation would not be one owed to any third party. So it would have to be one you owe to her. But it is questionable that you owe her such disclosures. To see this, consider that, if you owed her such disclosures, then she would owe you such disclosures as well. Now ask yourself whether such disclosures from her would really be in your interest: Would you want to know what she fantasizes about when the two of you make love? Would you be happier if she gave you this information, or do you think she would be happier if she gave it to you? If the answers to these -- yes! -- empirical questions (see Q866 and answer) are negative, then it is hard to see how she could owe you such disclosures that you would not want and that would be of no benefit to either of you. And then you should assume that you don't owe her such disclosures either -- unless you have special reason to believe that she would want them or that she or you would benefit from them.

In my reply to Tom's reply, I asked for argument, reasons, what philosophers are supposed to do as philosophers, if not duty-bound to do. And, finally, he did it. Thank you, Tom. Maybe I am thick and hence couldn't read the argument(s) that really did exist between or amongst those three short sentences of the original reply. But if I couldn't see it, did SteveB?