Myth is not the same thing as fiction.
And neither are appropriate designations for the genre of the gospels. Nor would they be equated to myths in the ancient world, even by non-christians. One of the earliest records we have of a pagan maligning Jesus explained the would-be miraculous birth by repeating a claim going back perhaps as far as Jesus himself that his father was a roman soldier and he was an illegitimate child. The same critic also compared Jesus to mythic heroes, but not to say they were equivalent, only to say that the stories about Jesus' miracles weren't particularly special. Indeed, we can see similar stories told about Pythagoras, Alexander the Great, Apollonius of Tyana, Caesar, and others.
Mythos didn't mean myth. But what we consider myths of the Greco-Roman world (e.g., the Homeric epics) were similar in ways that don't hold for the gospels. The gospels, for example, aren't in meter. They don't situate the stories they tell in some ambiguous time long, long ago such that nobody can check the facts or would even care to, as the reception of myth was not about the details (the most famous version of Medea's story, in which she kills her own children, was an addition added by Euripides), but about both entertainment and the transmission of culture through a collection of loosely connected, often contradicting body of stories connecting the present to the distant past (usually linking the present to some ancient founding hero/warrior, demi-god, or actual god, or multiple warriors, demi-gods, and/or gods).
The gospels do have literary parallels in the ancient world. In fact, the same stories about Alexander the Great or Caesar which include miracles and so forth are types of ancient biographies, and fall into a (rather nebulous) genre to which the gospels belong.