I honestly have a really hard time believing that sex came about entirely after the Fall.
You're not the only one.
I actually thought I was. Every time I've heard or seen it brought up by Orthodox (EO, I suppose), it was always consistent in saying that sex didn't exist before the Fall and likewise won't exist after the Resurrection. The notion strikes me as a bit Manichean, to which many EO seem inclined to lean toward anyway on these, and other, topics. The somewhat recent episode on Ancient Faith Today talked about this stuff, and I just found it unsatisfying.
I don't disagree with you. There's a danger in Fr. Josiah's thinking, but I sympathize with him because he is pretty much trying to feel consistent with whatever Patristic testimony he did his research from. The problem is that as you speculate further based on the present understanding of the biological and physiological functions of the human being, you could end up saying in the Kingdom, after the General Resurrection, all of our flesh may never really be needed. Fr. Josiah mentioned we'll probably rise from the dead without reproductive organs, gastrointestinal organs, or eyelashes (I distinctly remember these particular systems he said, and maybe sweat glands too he also mentioned). I find it troubling if you take this speculation to a certain extreme. One of the callers into the program even mentioned that we don't need our eyes in the general resurrection, and so that too will be done away with.
Where does one draw the line? Beats me! In heaven we probably don't need our bones, our nerves, our muscles, or our skin, because they only filled a physical purpose in our fallen nature
... or maybe the absurdity in all of this does need reevaluation of thinking, and I hope Fr. Josiah would be a bit more informed as to the strange implications of his views.