She realized right then and there that she did, in fact, want to marry Oliver, and they should do it now! Right now! While another couple was also getting married! She and Oliver didn't even have their own vows, because they couldn't compete with Barry and Iris. So they not only stole their ceremony, but also essentially stole their vows.

Barry and Iris just stood there, saying nothing, while Oliver and Felicity blabbered on about what they were doing, with no one ever pointing out that Oliver and Felicity could have Diggle marry them literally any time they wanted, any other time that was not during the post-funeral wedding of someone else.

But since no one pointed that out, they just continued on with it, and both couples were married together for some reason. And it was clearly supposed to be cute and heartwarming but instead it was just SO RUDE.