We're in agreement here! I'm glad. A good theological question for us to ask, next, would be: what is meant in our conversation by "inconceivable" as it pertains to biblical exegesis and theology generally. In a manner of speaking, not all inconceivables are created equal.
I'm curious where this is in the Small Catechism. I've looked and haven't found it yet. Nevertheless, once again our distaste for something is not the measure of good theology. I concur with Martin Chemnitz when he writes in the Formula of Concord (Solid Declaration, Art. 1.8): Reason does not recognize and knows nothing about the nature of this inherited defect. Instead, it must be learned and believed on the basis of the revelation of Scripture.
To us entering this world as enemies of God, your distaste is not of Lutherans or Luther at all but of Paul. Ephesians 2:3 calls all people children of wrath, including Christians prior to the in-breaking reign of Christ in the gift of the Holy Spirit in their lives. That is, children who are servants of the "prince of the power of the air" (i.e., Satan). We cannot be servants of two masters; we will love one and be an enemy toward the other.
That is one way of speaking about Sin. I quote (at length) from Beverly Gaventa's Our Mother Saint Paul (pg 129-130) for another image:
In [Romans] chapter 6, Sin's enslaving grasp comes into full and unmistakeable view. The chapter opens as a rebuttal of the possible conclusion that God's grace permits antinomianism: "Shall we persist in Sin so that grace may increase?" (6:1). Paul responds with an extended contrast that plays on the language of life and death. Those baptized into Christ's own death are simply dead to Sin - its power is shattered. All ambiguity falls aside in the second hald of the chapter, where the image of Sin as slave-owner is explicit. Sin was formerly the owner of these slaves (6:16-20). In that condition, they were enslaved to "impurity" and to "iniquity" (6:19). The only possible outcome of this slavery to Sin was death (6:20-22).
Not only did Sin enter and enslave, but also Sin's resume includes the unleashing of its cosmic partner, Death. When Paul introduces the extravagant contrast between Adam and Christ in 5:12-21, he connects the entry of Sin with that of Death: "Death came through sin, and so death spread to all because all have sinned" (5:12). Death itself "exercised dominion" (5:14, 17). Death is the very "wage" of Sin (6:23). Not surprisingly, the language of Death is... more extensive in Romans than in Paul's other letters. Elsewhere Paul may anticipate his own physical mortality as the opportunity to be with Christ (Phil 1:21; 2 Cor 5:1-9), but here Death is a force unleashed by Sin (see also 1 Cor 15:56).
Bringing this back to the subject of this thread, the implication of evolution prior to the Fall is that physical mortality while perhaps categorized as "not good", ought not to be categorized as part of Sin's reign over humanity; at least not as humanity and other species experienced mortality before the Fall. Post exile from Eden, however, mortality becomes a place where the rule of Death and Sin is most stinging. But on account of Christ, our mortality is transformed/redeemed into the moment where we gain the fruition of salvation.