As for dwarven monogamy itself, I'd like to see it made less rigid than it is currently, but still far more strict than the serial monogamy (to say nothing of infidelity) of real-life humans. In general, I think dwarves should stay firmly attached to one partner--but if the partner dies or causes deep feelings of betrayal, the relationship should end and a period of 'mourning' should begin, during which time the dwarf will not seek romance, and rebuff all advances. A widowed dwarf will mourn for a period 50% as long as the couple was together (as lovers + spouses, as applicable), and the 'wronged' party of a divorced couple will mourn for 20% of that time. (The 'guilty' party might not mourn at all.) This would almost completely eliminate the "Oh you kissed me right before you marched out to your death so now I'm permanently condemned to a loveless existence" problem that we have now, thanks to the strictness of DF's romance laws.
For specific dwarves, I think the existing personality traits of Love_Propensity and Lust_Propensity should be augmented by a new trait, Promiscuity. Love_Propensity would determine a dwarf's willingness to let their loved ones into their hearts, Lust_Propensity would control a dwarf's desire to get into their loved one's pants, and Promiscuity would influence the number of loved ones a dwarf would want to have at a single time. A very high value of Promiscuity would lead a dwarf to cheat with multiple partners, even in a perfectly happy marriage, while an extremely low Promiscuity might cause a dwarf to direct all romantic and sexual impulses toward just one other--even if that other is already happily (and very exclusively) married, or not even living in the same fort any more.
Getting back to real-life biology for a minute: In general, males feel cheated on when their mate has physical sex with males OTHER than him--thus increasing the chance that the offspring she bears will not carry on the male's genes, so that the time & energy spent on courting her was wasted, not to mention the fact that he might be using HIS energy to help support some OTHER male's offspring.
In contrast, females primarily feel cheated on when their mate's emotional, rather than physical, attentions are directed at females other than her. He can inseminate all the women he wants to, that's not going to affect the genetics or welfare of HER children . . . but if he starts spending TIME with another woman, forming an emotional bond, then he might leave his spouse & devote the majority of his time & energy raising the kids he's had with this OTHER woman, leaving his original spouse (and their children) out in the cold.
For this reason, a husband might divorce his wife if her high Promiscuity and Lust_Propensity lead her to do the deed with another man, and a wife might do the same if her husband's high Promiscuity & Love_Propensity cause him to spend most of his free time with women other than her. For homosexual couples . . . I dunno.
As for polyamory, and homo/bisexuality, each dwarf could have these as personality traits: Promiscuity and one other. (You may argue that a single variable could not possibly encompass the wide array of genders & sexualities, like "effeminate male who is attracted to masculine females", and you'd be right--but since these are dwarves, and all dwarves are pretty darn masculine, I think we're fine using a single value, at least for now.)
Each civilization would have these same traits, randomized at worldgen: One for that civ's acceptance of citizens engaging in free love, and another for that civ's understanding of homosexual relationships. (Each civ's Promiscuity must be kept above a certain minimum, and Homosexuality below a certain maximum, or the whole civilization would likely die out in just a generation or two.) As each child is born, his stats in these traits are modified (to some degree) by the culture into which he was born--he will be expected to conform to society's standards, leading him to possibly feel confined by constrictive rules, and/or disgusted by the depraved antics of others.