To answer this question first I need to know the situation, who is the person?
Not every person I meet is important to me, therefore not every utterance that happens to fall upon my ear is worthy of my acknowledgement let alone debate! In the situation you describe I get the feeling this person is a casual acquaintance or work colleague.
To me this person would rank very low on my "person who matters to me" list.
I did not transition to be a cause. I transitioned to save my life. My transition is 100% about me and my physical, mental and emotional well being
I personally do not have the energy, drive or inclination to try to create utopia. I have enough to worry about with my life and the people in it.
I try to let the world be. In my own life any people such as the one you describe would likely be someone I have not had to endure for at least 8 years now. They and whatever hurtful or ignorant thing they said are either a dim memory or are lost to time.
There is nothing I can say to someone who knows better. Efforts to change a persons closely held convictions are usually fruitless.