The bottom line for Catholics is that the marital act, as God designed it, allows humans to image the Trinity. It’s an incredible gift, when you think about it. Intentionally removing the procreative element from the act however, shatters the image.
In the situation you reference, we can tell you is that this couple is dealing with grave matter, as Seatuck said. What we can’t tell you is how culpable they might be of commiting mortal sin.