After Finny's comment, Gene doesn't really "decide to give it a chance" at all at that point. Gene acquiesces and simply allows Finny to believe that he agrees. This is yet another facet of Gene's betrayal, the lack of true connection, of honesty in communication between the two of them.
Gene hears and evaluates Finny's comment about love, but decides that Finny is persisting in his fantasy world. Gene admits that "in his seventeen years of life" that idea of loving something and it loving you back "in whatever way it has to love" as Finny states, is simply not true.
But rather than engage Finny in an honest and open conversation on this topic, he apathetically dismisses Finny's statement as "every other thought and belief of Finny's", as something that "ought to be true." This shows an element of direspect from Gene.