So here is an argument I've been meaning to bounce off someone for a while, yet my circle of friends seems to lack the necessary adversaries for it.

I'll argue this from a catholic perspective of a somewhat "progressive" kind.

Regarding the trinity teaching, theology asks at least two main questions: How is it to be understood, and why is it necessary (as opposed to a contingent feature).

The first is usually - well, at least in the classical scholastic teaching - answered by maintaining that the three theist personae are of the same essence/substance (homoousios), and analyzing the differences between them as a difference of relation. The first is necessary to not let the trinity dissolute in a tritheism, the second to maintain a trinity, rather than to morphe into a "strict" monotheism as in mainstream judaism/islam.*** The father is identified by the relation of begetting (begatting? gotting?) the son. The son by being begotten by the father, and the spirit - Hi Filioque - by proceeding from both Father and Son.

Fine; make of it what you will.

The point is, now, that according to classical scholastic theology, there exist two major metaphors to explain why such a construction is necessary. One goes back to Augustine and centers on the notion of self-knowledge; another was developed by Richard of St. Victor and uses the notion of love: For love to exist, one requires someone doing the loving and someone/thing towards which the love is pointed, i.e. that is loved. but, and this is St. Victors point, pure love cannot be conceived in this dualistic fashion as it transcends boundaries. An everyday way of expressing that would be that true love doesn't hold prisoner and in order not to be "exclusive" must be opening itself to transcend a dualistic configuration.

And now, obv., the Poly can claim, that this is precisely what he is doing. What would be the conservative christians theological, non-lame (a la "but they just wanna screw"), non-trivial ("but its written in the bible"; "the pope sez so") counter?

***It's an interesting side-bit that there are mystic traditions in both islam and judaism that also arrive at a concept of monotheism that allows/requires a innertheistic differentiation of sorts.

I think it should be pointed out here that the main metaphor for intertheistic love in the New Testament is not between a man and his wife, but between a father and his son. So the Trinitarian love is not conceptualized directly as a sexual love, lessening I think the force of this argument. In fact, in the New Testament the main marriage metaphor is that between the Church and Christ, which is a dualistic and exclusive relationship.

For a more direct argument, I would imagine that more conservative Christians would just deny that polyamory is truly a case of loving others. I think more typically in the Christian tradition sex is viewed as an act of selfishness or self-love (Kant also had this view). It only becomes an act of love towards another when also bound up in the promises and commitments of marriage.

Bolded: I didn't claim it was. Note also, that I'm not arguing on biblical grounds, but take up an argumentative figure by a scholastic theologican, which is standard exam-material for a dogmatics exam these days. Hence, it's not quite as simple as saying: "The NT construes the terms differently, hence it doesn't hold force." Richard doesn't found his concept on an idea of sexual love either, nor, for that matter, on a conception of paternal/parental love. The basic point is that Pure love is non-dyadic (which parental love is, for example).

From a post-Vaticanum II point of view, however, it's hard to see how any conception of Perfect Love cannot include some notion of sexual love too. Be that as it may, this is not what the argument relies on (at least I don't see how it does).

This is basically what I called the lame argument: Polyamory is just a way of rationalizing the urge of getting into others pants. True love only exist within marriage and two people.

For one, this fails on empirical grounds (unless you want to claim, for example, that the Mosuo and other cultures who have non-dyadic man/woman marriage conceptions know no love), for two it also fails on theological grounds - that's the whole idea: Perfect love is not a dyadic relation. (I guess the third alternative is to claim that pure love is indeed dyadic and that Richard was simply wrong to use the notion of love in the first place.)

“Do you want to know what goes on in the heart of the Trinity?
I’ll tell you.
At the heart of the Trinity
The Father laughs, and gives birth to the Son.
The Son then laughs back at the Father,
And gives birth to the Spirit.
Then the whole Trinity laughs,
And gives birth to us.”
-Meister Eckhart

Less metaphorically, he says:

"Every action of nature, morality, and art in its wholeness possesses three things-something generating, something generated, and the love of what generates for what is generated and vice versa."