If you know your Bible and your ancient myths, you'll know that the idea of fertility is central to pretty much all of Western thought, right up into the modern age. Simply put, fertility is the quality of being able to reproduce or give birth. In other words, a form of life without fertility isn't able to create future generations. For Pound, the modern world's inability to create anything beautiful is a type of symbolic infertility. Sure, we have great works of art from the past; but without making any ourselves, we're not making any meaningful contribution to the future of humanity. We're just looking out for ourselves and our money.
Line 43: It's not until the closing lines of Canto XLV that Pound introduces the symbol of fertility. Up until this point, he's conveyed his message by talking about a decline in craftsmanship and art in general. But here, he goes one step farther and says that usura "slayeth the child in the womb" (43). Okay. He probably doesn't mean this literally. But if you think about a great idea or a great piece of art as something that is born out of your imagination, Pound is saying here that the child—so to speak—of your great idea dies before it ever amounts to anything in the real world, since there's no reason for you to pursue good ideas unless they can make you some money.
Line 44: When Pound says that usura "stayeth the young man's courting" (44), he's basically saying that the world of modern finance and moneylending keeps young men from asking young women out on dates (or even for their hand in marriage). Now again, he doesn't necessarily mean this literally. But he's suggesting here that beautiful things like love and birth are corrupted by a world obsessed with money. After all, you can't spend much time thinking about much else when you're busy trying to make money.
Lines 45-46: In case he hasn't made his point enough, Pound claims that usura "hath brought palsey to bed" and that it "lyeth/ between the young bride and her bridegroom" (45-46). In this sense, palsey means paralysis. So Pound is saying that usury brings paralysis to bed and lies between a young husband and wife. In other words, he's saying that usury and modern finance makes a young man impotent, incapable of expressing his love for his wife sexually, and vice versa for the wife as well. Again, Pound might not be literal here. What he's saying is that all the most beautiful things about human relationships are totally ruined in a world where people's sense of value is determined by money instead of how beautiful something might be itself.