In this particular case, I think people side with Oliver because Felicity was out of line. Yes, adult couples share things, but Oliver had just found out he has a son. That is huge. Keeping things quite before being 100% sure the kid is really his makes perfect sense, needing some time to deal with the news, and see how things go is normal too. What is not normal is to track your boyfriend, demand someone to give you information and then act like you're the victim. The first time Felicity knew Oliver was a dad 15 seconds after Oliver, he barely had enough time to go talk to Samantha before Felicty made it all about her, and how Ollie was the worst because he needed some time to think before sharing such huge news.
The second time it's Oliver being an idiot because he could tell Felicity and Samantha would never know, but the first time Felicity was the one who acted like a child.