Ultimately, the question is, why do we have marriage at all?
Bruce Schneier, in his book "Liars and Outliers", discusses marriage as a solution to a problem in security. In a tribe of hunter-gatherers, a man who leaves his woman with the tribe while he goes off hunting leaves her available for other men. Result: He winds up raising someone else's children.
Solution: create an institution by which sexual exclusivity is encouraged and enforced by society at large. Thus, marriage. One man has an exclusive claim on one woman, and can expect society to support this claim.
We may say that modern society and modern technology has eliminated the need for this sort of mechanism, and nowadays marriage is all about love and life partners. (cont'd