Sure, the author's will to drag the manga on plays a part in Tarou's refusal to see the truth; but, differently from many other stories, there's a totally legitimate in-plot reason: he knows that, if the ?girl? he loves is really a girl, Eva's fated to disappear soon. Were I in his shoes, I wouldn't think too hard about his own suspicions, either; not when finding proof means literally killing what's clearly a sentient being, as immaterial as it is.