The complexity of a polyamorous relationship actually makes it more stable if you look at it in terms of the group relationship and not in terms of the individual relations within it. In a triad. a person who is currently dissatisfied with one partner still has a healthy relationship with the other. One has to be dissatisfied with with the relationship as a whole to decide to leave both partners.

That simply isn't what I mean when I talk about stability. A partner is still a person of roughly the same size and importance when there are others in the same reference class, and eir entrance into or departure from my life is an event of similar significance.

I feel that I am naturally monogamous - or possibly just patterned after my parents, who as far as I know are monogamous with one another. But I think that it would only be moderately difficult to perform the mindhacks to be comfortable with some types of polyamory, if the practical obstacles (e.g. how to deal with eventual children, prevent disease, present to the outside world, etc.) were all taken care of to my satisfaction.

I've been in a heterosexual relationship wherein I (but not the other party) had standing permission to have sex with other women, but I didn't find myself in a position to exercise this option in practice. (I did hit on a girl during that relationship, but she was located out of state.) This did not seem that difficult to adjust to psychologically. Possibly, this is because I attached no particular romantic emotion to hypothetical girls I could sleep with; they would serve as the functional equivalent of boyfriend-approved sex toys (whose needs and preferences would be more salient, because of course they'd be people, but nevertheless, they wouldn't occupy the same central role in my mind as an actual girlfriend would.) It's also possible that I would have freaked out completely if I'd actually had the opportunity to have sex with a woman, but this seems unlikely.

My suspicion is that I could also potentially be happy in a polyfaithful stable triad if all of the sub-relationships were virtually or completely free of drama, but more people than that, or appreciable amounts of romance/sex between triad persons and non-triad persons, or more conflict than "almost none", or any other complications, and I'd want to abandon the entire mess and find myself a nice singular partner.

I suspect that part of this inclination in myself is that I want my relationships to be permanent, reliable fixtures in my life. (I haven't managed this yet, but it is a very stable want.) Polyamories of any kind are necessarily more complicated. There are more practical obstacles, more negotiations, more neologistic rules, more outside perplexity, more spawning points for drama, more ways in which the relationship changes over time, and - if the parties involved are all inclined towards polyamory in the first place - more affordances for dissatisfaction within the established limits of the relationship. All of these things make the relationships less likely to still be around for the long haul in a more or less recognizable form, and that's not a desideratum I could give up nearly as easily as "all of my partner's nookie is for my exclusive use".