My boyfriend told me he could have chosen other girls who are prettier than me, but he still chose me, so I should be grateful, what does this mean?

Is it a normal sentence or should I be offended about it, I felt weird when I heard this sentence and I'm unable to get over it. Am I overreacting?
My boyfriend told me he could have chosen other girls who are prettier than me, but he still chose me, so I should be grateful, what does this mean?
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