According to Catholic teachings, sex is only allowed (not a sin) with some strict conditions: being within marriage, no contraception, etc. And, sex without fulfilling these conditions is usually a mortal sin. By contrast, many other things are often considered only inappropriate or venial sins, such as eating too much and being lazy (well, to make eating too much a mortal sin, you may need to eat so much that you come into a life danger). And, now, technology can produce sweetness without sugar (for example, zero sugar cola) for people who want to enjoy sweetness without gaining much weight, and this is not sinful at all, although this also seems to be contrary to the goal of sweetness. By contrast, the reason why masturbation and contraception are sinful and are usually mortal sins is purportedly that they are contrary to the goal of sex.
Besides, Catholics who are in celibacy for their whole lives are often given more admiration and praise. For example, if a Catholic saint is a virgin, then the fact that he or she is a virgin is often specially mentioned, seeming to make the saint more praiseworthy. By contrast, if the saint is just on a diet for the whole life, he or she is unlikely to be specially praised for that.
These facts lead me to wonder: Is sex ultimately bad, but just tolerated with specific conditions in this world? Is this the ultimate reason why sex is restricted strictly?
I hear that (perhaps from St. Faustina’s vision, but I’m not sure) if there is no original sin, then there will be no sex, and humans will reproduce in some other way. So, perhaps sex does not correspond to God’s plan at the beginning, but is just tolerated after the fall, somewhat like polygamy in the Old Testament?