Some Like It Hot (1959)
A:      Osgood Fielding III (Joe E. Brown)
B:      Jerry/Daphne (Jack Lemmon) 

A:       I called Mama.  She was so happy she cried.  She wants you to have her wedding gown.  It's white lace.
B:       Osgood, I can't get married in your mother's dress.  She and I-- we are not built the same way.  
A:       We can have it altered. 
B:       Oh, no, you don't!  Osgood, I'm gonna level with you.  We can't get married at all.   
A:       Why not? 
B:       Well, in the first place, I'm not a natural blonde.
A:       Doesn't matter.
B:       I smoke.  I smoke all the time.
A:       I don't care. 
B:       I have a terrible past.  For three years now, I've been living with a saxophone player.
A:       I forgive you.
B:       I can never have children.
A:       We can adopt some. 
B:       Well, you don't understand, Osgood!  Oh!!!
[Jerry removes the wig.]
B:     I'm a man!
A:     [without skipping a beat] Well, nobody's perfect.

 This line is #48 in AFI's 100 Years... 100 Movie Quotes.