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If only I had married someone else.
Literal
Different man [with-と] marry do-even-[if only-さえすれば] was-good [despite-のに].
~しさえすれば emphasizes 'if only I had just done X' — the さえ adds a 'just/merely' nuance to the ば conditional, stressing that the single act would have been enough. The trailing のに closes the sentence with counterfactual regret: 'if only X — but it didn't happen.' A compact construction for retrospective wishing.