It is greedy to marry just for money.
Lisa has been dating Bill mostly because of his money and wasn't very serious and now Cam asks her to marry him.
Lisa wants to respond to Cam's proposal.
Lisa tells Cam that she thought they were in a fling, and she doesn't feel like she really loves Cam so no.
Lisa tells Cam that she would love to marry him while thinking that she's probably set for life now.
Lisa saves herself from a lifetime of trouble and loneliness within a marriage by saying no.
Lisa ends up having a miserable marriage with Cam because they actually have zero chemistry.