The only real externalities mentioned are that
1.) You cant keep up with Jones if Jones have two incomes and you have one (It kinda assumed that if we both have to work it counts as external obstacle to marriage. E.g. the assumption seems to be that people want 1960 middle or higher class marriage and don't bother when they cant get exactly that).
2.) No one arrange "suitable" marriage for you and there is no societal pressure to marry for you.
I would say, that if you need pressure to marry, then maybe you do not want to marry. I would also say, that both points are you making marriage decisions based on social status it brings to you, in which case I am quite happy you are not my partner.
