I think my answer to the questions posed by SGB lies in her phrasing. I don't think of them as myth systems. I think of Christianity as the truth. As I've said before on this board, God is beyond complete human comprehension, but we have glimpses of him in the various religions. I believe that Christianity has more glimpses than other faiths, and more of the truth. But they all know something about what God is.
So it is not simply a case of discarding a myth-system that I happen not to like very much for another one, as I might pick up or discard a magazine or novel. Instead, being part of the church is being part of Christ's body on earth, a body that is trying to embody the truth of God in an imperfect, temporal, human way.
Like any institution, any particular kind of Christian church has its flaws, blind spots, and broken bits. Whether you stay in any one bit is about weighing the ways it represents the truth of God against the ways it fails or betrays that truth. For some people the issue of sexual freedom will be that tipping point, whilst others will weigh that against the profound truths the church teaches about sacrificial love. Leaving a denomination can be agonising, because it isn't simply a consumer decision but involves deep commitments of the intellect and the emotions.
And for the record, I was brought up as a staunch atheist, and have only ever attended a church of my own volition and by my own decision. I came to faith as part of a long process that involved my intellect as much as my emotions, and was both about experience and the assent of reason.