Your postulate that a third party must exist in a relationship of mutuality and care to be healthy is completely arbitrary and a non sequitor.
You view, then, as unhealthy, sex between a married couple who cannot conceive a child whatever the reason might be? Aristotle was liable to err you know.
http://www.youtube.com/watch?v=U0kJHQpvgB8
Opposition to the occasion (regardless of how likely that occasion is) of a third party is a form of selfishness, which represents an imperfection in the love and self-giving of the act.