Why would a guy said “I plead the 5th” when stating if a girl is pretty?

Anonymous
He was walking and said how this married woman is awesome. The convo

him- man she’s pretty awesome
me- awww u think she’s pretty
him- huh
me- she’s pretty?
him- uhhh I plead the 5th
Why would a guy said “I plead the 5th” when stating if a girl is pretty?
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