What did he say back to you?
"Well I'm free to wear what I want. All it is, is supporting our President. He's your President, too."
To which I replied, "True, you can wear whatever you want, but just in case you want to consider other people, please know that it makes many uncomfortable and is hurtful. I'm not saying you don't have a right to express yourself. Just be aware that you are in a society where other people are involved."
Then we parted, and a little further ahead an employee asked what we were talking about and he pointed at me, saying "that guy over there was calling my shirt offensive." Which I knew would be how he perceived it, so I walked over again (mind you, neither of us were getting worked up so no one was looking for a fight or anything) and clarified, "Nope! I was very careful with my words and I never said offensive. I just said it might be hurtful to people who care deeply about children and a host of other issues."
To which he mumbled something dismissive, and we both bid each other a good day and went our separate ways. I often go grocery shopping there with my children, and if they were there I probably wouldn't have said anything (my oldest one is 6, so I don't really want to introduce themes like hatred and bigotry yet) but I happened to be there by myself.
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Oh, funny story: another time I was at another store and saw someone with a red MAGA hat checking out right next to me. We were in an organic food market with a lot of young progressives, in a somewhat diverse area, and this was an older gentleman, and something about the situation really got me heated. I turned to get a closer look and confirm that he was really wearing that hat when I read the text more clearly: MAKE AMERICA GAY AGAIN.
Got a good chuckle out of that one, although I felt bad for the people just seeing his hat across the street. It was red and had the same typeface, so it might've been a little too subtle. :/