All along I thought the default font of iPhone’s Mail was Times New Roman.
It wasn’t. It was the default font of the signature.
Crazy.
So for those who don’t like accidentally sending HTML email from their iPhones: Clear out your entire signature in your iPhone and type a new one. Be careful not to use paste because the iPhone can copy and paste font styles from some apps.
P.S.
You can edit your signature in Settings > Mail and scroll down to Signature. But you probably already know that.