Non-marriage of female FMs happens quite often, especially if your empire goes through a FM boom, especially if your male children come of age in the span of a few years. You may go years without births or marriage proposals after that for the female children. Unless you expand dramatically. If you play a slow-paced game this will be less of a problem than when you try to blitz the entire game.

For a large empire, the ratio of FM / provinces will be lower as well compared to smaller factions. Thus a challenge with expansion is that you will have relatively fewer family members to go out on conquering sprees or governing difficult to control cities.