I dunno, I can see Tom wanting it to be true badly enough that he doesn't see the lie. He, after all, doesn't really know her that well; if she's being reserved and kind of standoffish and always sort of talking around the idea of them as a couple rather than straightforwardly saying "I dig you," perhaps that's just how amazons in love are? After all, she did give him this peach pit and she did take him to meet her mother, and that's big deal stuff, right...
Plus it would be kind of an interesting reflection. Diana can't really lie for shit, but she's also very difficult to fool; it would be sort of fun from a literary perspective (and not really inconsistent with his Squad days, IIRC, which I may not) if he were correspondingly an awesome liar but not necessarily that sensitive to when *he's* being duped. Though that would be more meaningful if they were intended to be a long-term pairing, which it now seems they are not. Pardon my glee.