The sources I’ve read say that according to Thomas Aquinas, human nature is characterised by certain natural dispositions or inclinations which need to be realized. Would you say that this accurately characterizes his view? The reason I ask is that it follows that whatever impedes the fulfilment of natural capacities is inherently evil. But this seems to prove too much. For instance, on this account the celibacy of a nun or a priest would be inherently evil, because it impedes the fulfilment of the natural tendency to procreate.
If the view I’ve described is correct, then it might illuminate a key difference between Protestant and Roman Catholic approaches to sexual ethics. For instance, it seems to me that it is good to have children, and we have a natural tendency to do so, but it does not follow that every sexual act must be procreative or potentially procreative.
As to your first question: Thomas Aquinas does hold that for any given nature -- such as a human being -- the good is that to which it is naturally inclined, that which its very nature disposes it to seek. So far, so good. However, the language of “impeding fulfillment” is dangerously imprecise. Thomas Aquinas doesn’t think we must all be having sexual intercourse all the time just because there is a procreative inclination, any more than he thinks we must all be eating all the time just because there is a nutritive inclination. Let’s work through this.
A power isn’t thwarted because I don’t use it, but because I don’t use it when I should use it, or because I use it at a time, or in a way, which frustrates its end or ends. Thus, going on a diet does not undermine the purpose of the nutritive inclination – but it would be would be wrong to overeat, induce vomiting, and then eat some more. Similarly, sexual abstinence does not undermine the purpose of the procreative inclination – but it would be dreadfully wrong to have sexual intercourse, but in a way which deliberately renders the act sterile. That would be something like eating and vomiting.
Now as to your conclusion about Protestantism and Catholicism: I think you are blurring the distinction between not trying to have a baby, and trying not to have a baby. Catholic ethics does not teach that my wife and must be trying to conceive every time we enjoy marital intercourse; we may be simply expressing the love of our procreative union. Nor does it teach that we may never abstain; but when we do have sexual intercourse, we must be open to the possibility which it carries. We must never intentionally close the door to new life.
Incidentally, you call this the Catholic approach to sexual ethics, but until the Anglicans broke ranks in the early 1930s, it was the Protestant approach too. Martin Luther agreed with it; so did John Calvin.