You are not over sensitive. However, the way you said to your spouse may be too harsh that got him responded that way plus any stress he might have during the day. His response is unacceptable anyway, especially in front of you too.
I've been to similar situations although my husband didn't response with that but he screamed and slammed the doors.
It is our mother instinct to protect our kids and it doesn't work the same for father.

Two things going on here. His response is unwarranted no matter what you said or did. You guys need to have a talk about that at some point for sure. It is likely a loss of temper, and definitely disrespectful. Does he do that at other times? To any other people?

The other thing, just for your future consideration, is examining how you told him to back off. Did you verbally go after him directly? "Back off our kid. Leave her alone." Or did you say it in a more courteous way? Hey, sweetie, Sally's struggling with this. Could we try again later? (and then you two discuss it later in private and work it out then). The first scenario would understandably inflame him (again, not condoning what he did in response), and it might help to try not to be as contentious. The second way (or something similar) is less likely to elicit such anger.

My spouse and I got into an argument about how he was treating one of our children. Basically he was trying to bully her into trying something that she did not want to do. She was afraid. I spoke up and said to back off. He flipped me the bird and told me to F*&! off! Never ever has this happened before. Marrried almost 15 yrs. I am shocked, hurt and pissed. I will always protect my children from being humiliated from anyone and I do not feel I did anything wrong BUT even if I did, the answer was way inappropriate in my opinion. I can not talk to him or even look at him because I am just too mad