Cylons and Sex

So when I was watching "Six Degrees of Separation", something occured to me:

In the episode, Helo and Sharon have sex, and as we know, Cylons' back "light up" when they have sex. Of course Helo at this point doesn't see her back, but it got me thinking. Wouldn't the Chief, at some point in time, have noticed Sharon's back when they were having sex? There are positions where it would be possible for him to see it (ex: doggy style).
But let's say that, for some reason, they never had sex in any position that would allow him to see her back, my next thought would be: what about Gaius?
Now, I will not believe that he did not have sex in many different positions with number Six. There has to be at least one time where he would have been in a position to see her back "light up". Not to mention that they did stay together for a while (in order for Number Six to get close to Gaius).

So I guess my question is: How is it that none of them realized that those women were Cylons?