People in love and soldiers in wartime are not bound by the rules of fair play.
Note: This proverb is frequently used when two people are contending for the love of a third.
all's fair in love and war
Any conduct is permissible in certain circumstances, as in Of course he called herall's fair in love and war. This maxim, stated in various forms from 1579 on, today sometimes appears altered by an addition or substitution, as in All's fair in love and the World Series, or All's fair in love and war and an election year.