Rejection, Mixed Signals, and a Hidden Relationship?

I sent a message to the person I was interested in, and he rejected it by saying he wasn’t interested. When i asked if i misread all? He replied: it happened to him before. After that, however, his body language showed the opposite. I therefore gave him ofcourse no attention. But i was confused with his earlier message... So, after doing some research, I found out that he is in a long-term relationship. Why would a man hide the fact that he is in a relationship? Why did he not wrote immediately to me, i am in a relationship, instead of not interested?

Rejection, Mixed Signals, and a Hidden Relationship?
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