Initially, it struck me that those aren't actually the two extremes. As I see it, the extremes would be 1) sex as recreation, the Planned Parenthood view (she was right about that extreme), and 2) the puritanical (i.e., prudish) "body is bad, sex is dirty" view. So using the "extremes" vs. "middle ground" lens, the Catholic view would actually be the "middle ground", falling in between these two unhealthy extremes.
But then I realized there was something wrong with framing this (or any moral issue) in terms of "extremes" vs. "middle ground". After all, the commenter herself said that the Catholic view of sex "makes sense" and even seems "wonderful". In fact, far from being extreme, the Catholic view of sex used to be the cultural norm, not so very long ago!
I believe we are using the wrong terminology, because we are looking at morality through the wrong lens.
I suggest a different paradigm entirely: Instead of shooting for some mathematical mean between the "extremes", why not instead speak of what is ordered vs. disordered?
Our minds understand order and our souls crave it, because where there is right order, things flourish, thrive and strengthen. Where there is disorder? Not so much.
Some things are still easy for us to recognize as disordered:
Rape is disordered.
Pedophilia is disordered.
Murder is disordered.
Lying is disordered.
Theft is disordered.
Physical or emotional cruelty of any kind is disordered.
Some things that are fuzzier for us moderns become clearer when we look at outcomes. For example, it's popular to act as if sex with multiple partners is ordered and natural, but reality shows us differently: Sexually transmitted diseases and infections are nature's blunt way of saying that human beings are meant to be monogamous. Also, 50 million dead unborn babies is a (blood-red) neon sign that our use of sexuality (and our mindset about it) is gravely disordered. There are many other such signs, too, if we have eyes to see.
We know innately that the virtues are ordered. Think of patience, justice, prudence, temperance, fortitude, charity, truthfulness, and so on -- all these represent moral order, not "extremes" to be dismissed for a "middle ground". Chastity has always been included in the virtues.
Our human dignity requires that we aim our sights at what is ordered and then strive for that. Will we always hit the mark? Hardly! The virtues are habits, and moral habits must be cultivated over time (sometimes a long time!), with the help of God's grace. The goal should always be to leave disorder behind and head towards order, which will bring refreshment and interior peace to the soul.
All that to say that from now on, instead of talking about "extremes" vs. "middle ground", I'm going to speak in terms of ordered vs. disordered.