Love vs. Lust

Meursault shows himself (unsurprisingly) indifferent towards marriage with Marie because he never cared about marriage enough to think about it, he does not care whether Marie and him end up marrying or not since the emotional characteristics of marriage never crossed his mind.

At this point, it is an irrevocable fact that Meursault cares more about physical stimuli than any type of event that requires feelings or emotions, he focuses on the practical details of everyday life. He mostly looks for happiness in things like cigarettes, sex or nature.