。
All's fair in love and war.
Literal
Romantic-love [and-と] war [topic-は] means [object-を] do-not-choose.
A classical-flavored form of the 'love and war' proverb. The は takes the combined topic 恋と戦争 directly rather than going through では. The sentence-final 選ばず uses the classical negative suffix ~ず (equivalent to modern ~ない), giving the proverb a stately, written feel.