Dude, I have no time for or interest in your squabbles with other posters. I thought you would understand on your own why I asked the question I did, apparently I was wrong.
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Since you seem honest in your question:
If you want to be pedantic it can obviously be argued that people are born asexual as toddlers have no sexual desires or preferences. As sexuality develops it seems abundantly clear that there's a strong genetic/biological component that most likely interacts with the environment any person grows up in.
The original statement you asked about "people are not born gay" is derogatory because of what is typically implied by people making that statement. It might be wrong, at some level, as it seems at the very least likely that there are environmental factors at play. So there's not a strict "born gay, grows up to be gay or born straight, grows up to be straight" dichotomy. So, if you want to be pedantic, you can say that people aren't born gay. However, there's not even a gay/straight dichotomy in the first place, sexuality varies along more of a continuum. Note that this doesn't imply a choice, and it seems likely to me at least that for some people the genetic/biological factors are very strong.
Most gay people will feel that they are "born gay" just as much as you feel "born straight". The statement you asked about has traditionally been uttered along with things like "it's a choice", It's not natural", "it's an illness" or "you can learn to be straight" etc. If you were in a minority as a straight person and there was a history of recent clear discrimination and still lasting discrimination to some degree in the society you lived and people had been using "you're not born straight" along with other terms like the one's I mentioned I think perhaps you would see why "you're not born straight" could be seen as derogatory.