system
Given the following post, respond as if you're commenting on Reddit:
human
[BSG] If the skinjob Cylons are supposed to be nearly biologically identical to real humans, why do their spines glow when they have sex?

Unless I'm missing something, I don't think this was ever explained.  Seems like this would be quite the dead giveaway of a Cylon detector if the colonials could just go ahead and sexually torture the Cylons to orgasm, and it seemed like a good portion of the fleet didn't have any qualms about torturing Cylons.
gpt
The Spine glows due to the fluorescing nerves that were originally engineered into the skin jobs (sj), as a way of debugging their nervous systems.  Most of the acitivating genes have been disabled, but under intense stimulation, parts of the sj Cyolns can be induced to fluoresce.  The fluorescing materials, being limited to the nervous system is hard to biopsy, and can be considered undetectable in the colonial fleet with they lack the available technology test without fully removing the spine or brain.

Since they were able to test fully organs.  It was established that Cylon bones are different from Human bones.  One can assume the difficulty the creation of a Cylon detector was not the lack of physiological differences, but the lack of the necessary advanced detection technologies on Galactica and within the fleet.   The original Battlestars were old dumb ships, with limited computers and a large compliment of analog systems.  In the pilot we learn that Adama is very much against technology the Cylons could use, and Galatica is about to be a museum, so they wouldn't be expected to have a state of the art lab.  The SJ were never detected before because no-one knew to look for them.   After they were discovered the remaining humans had to look for them with what they had, which was almost nothing.

The fluorescing genes were not present in the final five, just within the 7 SJ they created. All SJ are aware that this happens, and the stress of being uncovered by and the general hatred of humans was enough to keep the pro-active agents from enjoying themselves so much they got discovered.   The sleeper agents, were given mental issues to prevent them from orgasming.   The Six with Baltar let her self go (along with all the others) once the plan had moved to the point of no return and the need for secrecy of the SJ was removed.

I figured, if you are going to design a biological robot, with a conciseness that is transferable, odds are you are going to need a way to debug them, and to program the bodies. The reason that you have some glow is that fully disabling the fluoresces, would make it impossible for OMT (Organic Memory Transfer, ie Download) to write the mind of a dead Cylon into a new body. The difference between the 7 and the 5. Is that the 7 have it in there whole nervous system, and the 5 only in their brain. This being the case of having different design teams coming up with slightly different solutions to the same problem.  

Edit:  Flow, spelling and more from below.