Am I overreacting ?

I recently met a girl in the gym and sometime we talk when we workout. My husband also goes to this gym and he met her couple of times. She is really in shape and looks amazing. She is my age. My husband 's brother is looking for a girlfriend. I friend her on Facebook and husband copies gym pics and sends it to his brother. I forget about it. His brother still pays for his ex 's car and insurance. So yesterday they are having a convo I guess in the morning where husband is trying to explain to his brother to stop paying for her insurance. So I am at work. He sends me screenshot of this conversation where he tells the brother to stop paying. He didn't realize that on top of this screenshot he had a snippet of a conversation about some and he says exactly this: she is smoking, I guess I am attrackted to her, but I love Tanya (me) and to me she is the sexiest woman. So I get mad, like why are u so dumb to not read what you send me. He starts saying it was an accident and he is just talking about some actress from a movie. I know it's bs, it's definitely her. So this got me thinking. Why am I so upset over this? Men can like other women and not take it any further? Or do you think this is just not right? I personally find some men good looking, but I don't think I am attrackted to them and I certainly wouldn't send this chit to my husband.