Plot hole: The whole problem with the teleporter occurs when fly DNA is mixed in with Seth Brundle's DNA, starting his transformation into the Brundlefly. Brundle is on a hiding to nothing from the word go, and the fly is irrelevant. Humans are a walking talking mass of foreign DNA - we are host to one trillion bacteria all of which has a complete complement of DNA, as do various tiny mites that live in our hair follicles and all sorts of single cell organisms in our gut. If the transporter serves to mix the DNA of all living creatures which are in the transporter pod at the time Brundle would turn into a half-man, half-bacteria. Incidentally, DNA from a bacteria, an amoeba or a hair follicle mite would be just as 'compatible' with human DNA as that from an insect. It's quite a simple chemical, really.
Suggested correction: Being as how the bacteria and mites and such were IN or ON Seth, the machine was able to organize those symbiotic relationships accordingly as teleporting one would teleport all. The fly was separate, not touching. The machine was not programmed to anticipate two separated entities and so combined them into one on the other side.
Phixius ★
We leave behind a vapour trail of bacteria and viruses (among other things) as we walk, in our breath and emanations from pores in our skin, and Brundle isn't trapping any in his clothes as he isn't wearing any. Brundle has an invisible cloud of DNA floating around him in that teleportation chamber and as far as the machine is concerned their DNA has exactly the same status as that of the fly.