There is a certain school of thought that opposes monogamy on the basis that it restricts the amount of love, intimacy, and sex one can have. It considers love, intimacy, and sex terribly important and wishes to maximise them. In answer to that idea, I present this dialog:
Adam: Monogamy is restrictive.
Jane: So is everything.
Adam: Well what's the point of monogamy? Why is it a good restriction?
Jane: You've missed the point; if everything is restrictive in some way, we needn't defend against that charge by giving reasons something is worth the cost. Everything has that cost. Further, you act as if monogamy has something to prove. But it is conventional, and deviations from convention take effort. In this case, great effort. We should be monogamous unless you can show it to be such a loathsome way of life that we recoil from it, and prefer great hardships.
Adam: Monogamy limits my sex life; that's bad.
Jane: Why do you think sex important?
Adam: It is an expression of love and intimacy.
Jane: Monogamy/romance is a tradition that offers love and intimacy. It is their only known source. They are not creatures of reason, gained through argument. The specific criteria for feeling them has never been discovered. How do you hope them to remain when you scorn their brothers and sisters?
Adam: Maybe we can take part of the monogamy tradition and leave the rest.
Jane: That is a delicate matter. Maybe we can make some small changes. What specifically do you suggest?
Adam: I want to have the good and fun parts, like love and intimacy and good sex, and leave out the parts that limit how much of these I get.
Jane: Do you deny the limiting parts are an integral part of the tradition?
Adam: What do you mean?
Jane: The causal roles are unclear but we can see some surface relationships. For example, exclusivity is thought to make people feel more special and intimate.
Adam: You're suggesting the various aspects of the tradition are related in complex ways and I cannot expect them to continue to function in isolation?
Jane: That's right.
Adam: Could I devise a life support system so they continue to work?
Jane: Perhaps. But I imagine the first step to be creating explanations for how the parts you want to rid yourself of are related to the parts you want to keep. This will help you see whether your course is wise, and also see what sort of replacements are needed to retain functionality.
Adam: I'll get back to you.
Jane: Before you go, let me express my skepticism in the notion that half of the monogamy tradition is very good, and half is very bad, and that the proper course is to get more of one part by removing another part.
Adam: What do you think of love and intimacy?
Jane: I am skeptical of their rationality. This is not to say that I oppose them. But I don't see anything to be gained by seeking out as much of them as possible. Our traditions cause us to desire them to some extent, and also provide for that desire to be fulfilled. What's wrong with leaving that alone?
Adam: I'm not convinced the traditions say that, exactly. Why do you think I'm so interested in lots of sex and love?
Jane: There are many voices today, including hedonists and religious conservatives. Both make mistakes, and we needn't follow any voice exactly in order to be generally in the mold of convention. I think my interpretation is true enough to the tradition for its followers to draw on our cultural knowledge, and that my position would be considered reasonable by most. In fact, I think it would have a larger appeal than the hedonist position, because it is moderate and will not alienate any major factions.