The private desire to please oneself motivates apathy to any strict morality forbidding that self-pleasure. I suspect more so than any supposed rationality; see atheists and religion.
The article on Russell is actually quite relevant to the overall discussion on polyamory & unfaithfulness. The final quote by Russell on his realization of what eluded him in his life, nails it.
Through the long years
I sought peace
I found ecstasy, I found anguish,
I found madness,
I found loneliness,
I found the solitary pain
that gnaws the heart,
But peace I did not find.
Now, old & near my end,
I have known you,
And, knowing you,
I have found both ecstasy & peace
I know rest
After so many lonely years.
I know what life & love may be.
Now, if I sleep
I shall sleep fulfilled.