(note that this natural doctrine fails to prove homosexuality is sinful since it is known to exist among baboons, bonobos, and other primates, but Paul would not likely have been aware of this at the time).

Personally, agency is not the sole right of humans, but is also held by animals. Also, we humans, as the children of God, are held to a much higher standard in all things, so what animals do should not influence us.

At this point, one has to suggest that at least in practice (relating to the writing of epistles), Paul seems to have suggested that Levitican law should be followed or at least looked to as a source of moral edification.

He did, but in many places critisizes the saints for following it to strictly, as many of the laws in the Levitical Law no longer applied (like sacrifices). So, we must distinguish between the ones that were done away with and the ones that weren't.

At this point, wouldn't you agree that transgenic experiments regarding crops (or even hybridization) would be a perversion of normal natural sexual tendencies for plants as homosexuality is claimed to be? Or is the idea simply that any portion of Leviticus that Paul doesn't cite can be thrown out?

If using just the Bible I would agree that this is the only logical way to interpret things, and some Christian sects do so. Me, personally, I am not an average Christian, so I rely on other sourses as well. I believe I said this as a parenthetical in a previous post.