Before they were married Amanda and Winona knew each other, were childhood friends. they wished for there families to become one so they created a betrothal contract stating that if they had children they would marry. Problem: they didn't stipulate gender.
BONUS: if Spock agrees due to it being his mothers wish
BONUS: if Jim realising he can't say no, runs off hiding
BONUS: if Spock prime helps Jim but tries to surreptitiously change his mind
BONUS: if Spock or Jim can get pregnant, this can be because of technology, alien plant or genetics