01. None of a girls from previous chapters could produce legitimate heir.
02. Prince is the only son of a current king; girls doesn't inherit a kingdom.
Due to a rising power of kingdoms in North & South, we have two solutions,
if prince would want to refuse consummate marriage and unable to have a heir:
01. King abdicates his position, his brother Conrad becomes a king and marries a foreign princess for a dynasty.
02. King changes some laws, so one of princesses could be a ruler, her children would be from matrilineal marriage.
Oh, but what ~MC? - he'd remain a prince, but from that point on this story wouldn't ~ him anymore, so time to look
for another VN, where MC has a bit more of common sense and understands, how things works in medieval society.
P.S.: MC sisters barely could have his children, until king is alive and MC comes to power, but you've missed Cass route.