There is nothing wrong with throwing a party to celebrate something in your life. That's pretty much exactly what parties are. Sometimes the "thing" in your life you are celebrating is happening to someone else, sometimes its happening to you, either way, its a perfectly etiquette-following, acceptable, polite thing to do. Its never ok to request, or even really imply, guests must give you a gift though. You can ask they give a gift to someone else, but you can't ask they give a gift to you.
So for expectant parents to throw a party celebrating "whoo-hoo we're super excited to be pregnant and want to share our joy with friends & family by having a party" that's totally cool, gracious in fact. But once they slap on the word shower - party speak for you must bring a gift to shower the guest(s) of honor - its stops being ok, and is greedy. Because it means it's not about "whoo-hoo we're super excited to be pregnant and want to share our joy with friends & family" but rather "whoo-hoo we're super excited we're pregnant, now what can we get in gifts?"