指腹為婚

to propose the future marriage of two unborn babies on condition that one turns out to be a boy, and the other, a girl (idiom)

finger / to point at or to / to indicate or refer to / to depend on / to count on / (of hair) to stand on end

to marry / marriage / wedding / to take a wife

abdomen / stomach / belly

as (in the capacity of) / to take sth as / to act as / to serve as / to behave as / to become / to be / to do / by (in the passive voice)

because of / for / to