Very brief comment on Christian fundamentalism

The logic of conservative religious attitudes towards pleasure might be given their most generous expression by summarizing them as follows: pleasure without commitment rewards/accommodates selfishness and individualism. pleasure with commitment allows you to form deep ties of creation with others, because it breaks down the division between self and other, by encouraging consideration of the other's well-being. If my experiences lead me to tie my pleasure to your pleasure, then we can create broader and more intensive projects. Commitment means that the range of those projects are extended in time, in intensity as well as extensity. You can trust me to be with you in the long-haul, so we can both conceive of our projects for living together, drawing upon our mutual reserves.
This logic is not limited to conservatism, we can build it with equal (actually greater) consistency from Spinozism. A body formed of two bodies* in perpetual contact has more power and more power of action than a single body.
If you want to be really funky about it, you can talk about the significance of mixing genders, as in having two different systems of sensual pleasure interacting. Hence the fundamentalist preference for heterosexual couples could be expressed as a directive to increase the qualitative difference in systems of pleasure within a couple. This is too generous though, since it misreads the actual nature of homosexual expression (though it might be the same reason it is poorly constructed in psychoanalysis).
Either way, there is a qualitative increase in powers of action.
Of course, this argument could be used to justify a polyamorous community as well. These don't seem to occur very often with much longevity though. The level of attentiveness possible between three people is more difficult than among two. The biology comes in here as well- takes two to produce offspring, and the production of young is a consistent enough "project" to form a sort of attractor.
So there's a good argument for commitment, based on its capacity to wedge open ontological narcissism (to be sloppy with words); and also on a Spinozist logic of expanding powers of action.

*and using Merleau-Ponty or Bergson, "body" means the broad level of kinesthetic and emotive/intellectual experience tied to and conditioning the flesh, body-image for instance