Actually silicone products aren't that bad for your hair! BioSilk uses dimethicone and nobody has ever complained (that I know of) that it makes their hair break. The silicone in it simply coats the hair and form a very soft hold, which does not harm the hair. The outer soft "shell" of silicone around the hair reduces the amount of friction on the actual hair and results in less damage to the cuticle from combing, brushing, etc etc, and also helps the hair to lie flatter because of the reduced friction and reduction in static assosiated with it. However, hair spray and gel (which are alcohol based polymers) can make your hair so stiff that it will break (this doesn't usually happen but it can). You should be worried more about excessive alcohol/polymer based styling products such as those.
The argument that "it's for blow drying it straight not flat iron" is also partially invalid because blow drying can still increase the temperature of your hair significantly. In fact, if you blow dry too close, you could make the water in your hair evaporate so fast that it forms quickly expanding steam bubbles inside the hair cuticle which damage it permanently. This can also happen if you flat iron your hair while it's wet. Anyways, the point is that high temperatures are involved in both circumstances. If silicones were not appropriate for use with heat, then you would not be able to blow dry with them either. If you're not convinced, stick your hand directly in front of the blow dryer when it's on the highest heat setting.
Heat styling of the hair does break some of the bonds in the hair semi-permanently, (which may be why you say your hair is becoming less curly). This may be because you iron so often that the some of hair bonds dont have enough time to reform completely. Not all of the bonds may necessarily reform when you add water. This isn't necessarily bad however, because heat cannot generally cant destroy bonds, only break them temporarily, and they will always reform eventually, if they do not reform right away. Relaxers however are a different story. Anyways, just deep condition every so often to keep your hair strong!
What DOES make hair break, become damaged, and dull looking is when your flat iron is not hot enough, or when it's too hot, brushing your hair too much when it's wet, backbrushing, and not using conditioning products. Many people think that if it's too hot, it's going to burn your hair, so they put the setting too low (this is bad!). There is some truth to the "it'll burn my hair!" , but it takes a lot of flat ironing for this to happen (which is why you get hair trims every so often to get rid of damaged ends), or if you get your hair stuck in a curling iron.
Your hair can be damaged by a flat iron that is not hot enough because you have to run over your hair multiply times (which puts a lot more stress on the hair, especially the cuticle) and it becomes damaged mechanically. Invest in a high quality straightener (if you don't already have one)! However, a flat iron that is excessively hot can burn hair really fast. This shouldn't happen however since no products that I know of will heat up more than 425 degrees F. Hair burns at approximately the same temperature as paper, which is 451 degrees F, which is about 25 degrees higher than the hottest salon quality flat irons. However, fine hair may scorch at a lower temperature, so if you think it's too low (it's hard to straighten) and want to raise up the temp, test it on a little bit of hair first and closely examine the results before you tackle all of your hair with it. In general, with some modern hair appliances you buy at say . . . HEB, Hair will be damaged mechanically more quickly than with heat, which is why you should use a high quality flat iron that reduces mechanical damage to your hair.
Hair shouldn't be naturally dry looking! If you notice that your hair is getting frizzy and dry, make sure you are using the right temperature on your iron. You might have it too high or even too low! Use a product also to help with straightening and reduce friction. Don't brush your hair too much also, because it stresses your cuticles. It's OK for styling purposes though and it might even help cuticles lie flat and be shiny. But don't brush excessively when your hair is wet, but most importantly when it is wet and doesn't have quality conditioner; when it dries it can make frizzy problems worse. Always use quality conditioning products but be price conscious. If you compare ingredients, you may find that some products are exactly the same or very similar with exception to the brand and price! Always do your research. 
Happy styling!
-Kennieth
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