I think I understand the idea Eugene is getting at in the sibling thread. Let me see if I can explain it a little differently.

As Sister Y explained in this excellent article, people no longer have a way of committing themselves to marriage. This is a problem for two reasons, neither of which applies to vegetarianism.

1. In a sense, marriage IS commitment, and talking about a "marriage" without commitment is like talking about a "prisoner" who can leave his cell any time he wants, or a "warranty" which can be ignored at the company's discretion. Now, you could argue that this is a matter of semantics, and to some extent you would be right, but there is a deeper issue here; that marriage with commitment and "marriage" without commitment are so far apart in relationship-space that we should treat them as completely different things, and that we might be justified in not wanting to call these clusters of relationships by the same name at all (some people like to call the modern relationship cluster Marriage 2.0 for just this reason).

2. If you can't credibly commit to doing something, you are going to have trouble finding people who are willing to expose themselves to risk should you fail to do so. Thus, by removing your freedom to pre-commit yourself to fulfilling a marriage contract, your freedom to enter into these contracts has been reduced (indeed, the collapse of the marriage rate appears to be an empirical confirmation of this model). Thomas Schelling covered this in his The Strategy of Conflict.

Among the legal privileges of corporations, two that are mentioned in textbooks are the right to sue and the "right" to be sued. Who wants to be sued! But the right to be sued is the power to make a promise: to borrow money, to enter a contract, to do business with someone who might be damaged. If suit does arise, the "right" seems a liability in retrospect; beforehand it was a prerequisite to doing business.

Now, the term under discussion is "monogamy", not "marriage", but back to problem 1; the modern serial "monogamy" is a completely different cluster of relationships from the old monogamy, which implied marriage. Dalrock, for example, argues that serial "monogamy" is a promiscuous and immoral relationship model, which are things he doesn't believe about the traditional religious monogamy model. Whether you agree with him or not, the point is, again, that modern serial "monogamy" is pretty different from old monogamy which meant things like not marrying two wives at once, and maybe some people want to avoid overloading an existing term to incorporate such a different new concept.