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I think you mean (or he meant) "prude".
Prudent means someone carefully thinks things through or considers things, particularly regarding the future. This is typically a positive thing to be called.
Prude (which is what I think your ex boyfriend/you meant) means someone who is overly proper (kinda like a stick in the mud) or pretends to be way more proper or modest than they really are. When someone calls a woman a prude, it usually is negative and it's like they're telling you you're a "goody-two-shoes"0