There are negative social consequences to adultery related to group success, but I was saying that these consequences on your model do not seem to account for the deep sense of betrayal and sorrow felt by the spouse. In practice it's hard to square that discrepancy. Suppose, for example, that the adulterer makes recompense for the breach of contract, etc. It is unlikely that the offended party would find that finite (?) recompense satisfactory. If we turn to human experience there seems to be something more at play here.
I don't see it as an all or nothing affair... The person who approaches morality as a hypothetical will be significantly different from the person who approaches it categorically even if they aren't unequivocally above social consequences. Are you familiar with Plato's tale of the Ring of Gyges? Those who hold to a hypothetical morality approach it as an economical affair. On your model morality is a means to group success, which is a means to individual success. Thus there is already subordination of the group to the individual, and this naturally leads to a weakening of morality. I suppose a more modern example is
Quicksilver from X-Men. He is able to steal without getting caught, and the society is so vast that his predilection for individual interest doesn't undermine it.
It seems that our society's moral compass surpasses rational self-interest. Whether that is due to Christian influence, I do not know, but the 'evolutionary' explanation must needs bridge a gap. Perhaps it would say that the categorical phenomenology of morality is due to an inference so ingrained that it has mistakenly become a basic premise of its own. In any case, something like that seems to be necessary to square the two sides of the equation.