Three types of love are identified:
Eros / Romantic love (intensity, sexual interest, and engagement)
Mania / Passionate love (romantic love with obsession)
Storge / Companionate love (warm, less intense love, devoid of attraction and sexual desire.)
The chronologic sequence and research findings (My comments in parenthesis):
Passionate love - (This gets the couple together.) If it doesn't end soon enough, relationship satisfaction is decreased. Too much drama for the long-term.
Romantic love - (This builds and maintains the pair bond.) It declines later in all cases, but for 1 of 8 couples it remains high enough to maintain sexual interest and therefore continue.
Companionate love - Also called "friend love." Satisfaction with the relationship is lower than with romantic love. (This is where an overwhelming majority of marriages end up under the monogamy for life / one-and-only plan.)
From the paper:
Contrary to what has been widely believed, long-term romantic love (with intensity, sexual interest, and engagement, but without the obsessive element common in new relationships), appears to be a real phenomenon that may be enhancing to individuals’ lives— positively intense long-term romantic love sets a standard that couples (and marital therapists) can strive for that is higher than seems to have been generally considered realistic. This could also be distressing for long-term couples who have achieved a kind of contented, even happy—but not intensely romantic—status quo, assuming it is the best anyone can expect. Couples benefit from downward social comparison with other couples and will even distort their evaluation of their own relationship to an objectively unrealistically positive view.These last two sentences may explain the more extreme resistance to polyamory often seen: