Is it okay to not tell your partner this?

My boyfriend got asked if he was single n for his insta, he said he has a girlfriend and she walked away but this was when he was with his cousin. He didn't tell me about it till 5 days later and lied that nobody has ever asked for his number n stuff. I got mad, because he lied but he said that he thought it wasn't something worth mentioning n was "scared" that I would get mad and it would start an argument. What do y'all think?

Is it okay to not tell your partner this?
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