I'm an INFJ male. I know only one INTJ and I was semi-dating him. It was actually me who occasionally got his feelings hurt!
I completely disagree. For me, it's the other way around.It's very easy to hurt an INFJ if you try to hurt their feelings, but very hard if you're trying to be nice (even if you're very emotionally clumsy).
(for the sake of readability, I will refer to a he-person only)
When someone intentionally tries to hurt my feelings, he will never be succesful. Instead, I automatically (Ni on autopilot) analyze the situation and his need to insult me. Understanding where the lashing out is coming from really helps me to control my emotions. Instead of yelling back, I tell him why I think he reacted the way he did (and some then consider me psychic, but it's only about paying attention and understanding the human mind). It usually ends up in him admitting and realizing that it was indeed another thing altogether that was bugging him. Even if I'm wrong, it usually ends up well (us talking about it).
It is the unintentional things that can hurt my feelings. This is difficult to explain, so I'll leave it short this time. It's about me analyzing people's behavior and understanding that their actions and words do not match. Facades hurt. For example: saying "I'm too busy" time after time hurts, because there's a message underneath (since time is about priorization, I'm obviously not worthy of his time. It hurts to know there's something wrong, but the other person is hiding the real reason in a cowardly manner, making me look a fool.).
It's mostly meta level stuff with me. INFJs are said to be almost ridiculously fragile. Even a short, weird look from a classmate might hurt me (my Ni goes crazy with paranoia), unless I understand where it's coming from. As soon as I do, even if it's hurtful, it doesn't hurt one bit.