[Hermann the Cherusker]

When couples abtain in the practice of NFP, are they making a sacrifice "for the kingdom of Heaven", or is their intention not rather to avoid children for those just reasons which are admitted by the Church?

You would have to ask those couples. I can only say that I believe that gaining mastery over the sexual appetite is incredibly important for any advance in Christian virtue, since the sexual appetite is a part of the love of sensual pleasures that is concupiscence and which is the form of original sin. NFP can be used in this way by a couple if they use this as their motivation.

NFP does not have to be a part of ascetical struggle to be licit, but it does to be virtuous.

[gbcdoj]

NFP does not have to be a part of ascetical struggle to be licit, but it does to be virtuous.

Okay, that makes sense.