My boyfriend is going to leave me for accidentally injuring his best friend, unless I apologize when it wasn't my fault?

Anonymous
I have never liked my boyfriend's best friend, and lately we attended his boxing class together, where he was just getting in my face throughout. My boyfriend didn't stand up for me.
Well after the class, he jokingly challenged me to spar with him with which to his surprise I agreed. Now while I am not a avid sportsperson, the only fighting experience I had was when I took a female self defense elective in high school, with that stated, I wound up kneeing him where it hurts instinctively.
I had no idea that he would not be wearing any protective gear, nor did he establish that kicks were not allowed. We thought that he would be okay after a short rest as I barely touched him (it wasn't a hard hit at all btw) , but he was still throwing up and writhing after some time so we took him back home to sleep it off. He claimed his privates were still "in agony" by night, so we took him to a hospital where he was told he was going to lose one of his testicles because he waited too long to see a doctor, and he spent a week in bed with his legs in the air.
Now, he is insisting that I did it on purpose, and my boyfriend is backing him on it and has explained that unless I apologize to him, he will break up with me.
If anything, he should be apologizing to me for not standing up for me when his friend taunted and insulted me during the class! And it was a pure accident due to his personal negligence!
What should I do?
It is my fault and I should apologize.
Vote A
He is being childish and it is not my fault.
Vote B
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My boyfriend is going to leave me for accidentally injuring his best friend, unless I apologize when it wasn't my fault?
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