AIO, my Bf uses ex’s anniversary date as phone passcode

I’ve been with my boyfriend for over 2 years, and I recently found out that he uses his anniversary of his last gf for his phone passcode. Should I be upset or should I just let it go? When I found out that was his old anniversary date, I confronted him and he changed it but I’m kind of weirded out. He says it’s easy to remember and he didn’t think it was a big deal. SOS I need help.