This is not entirely germane to a spouse looking to outsource. Such people have their life partner and get many of teh advantages the unhappy poster cites for monogamous people.
I wanted to post this given polyamory is the hammer I pull out any time someone drops in here and explains their problem/nail. I'm only too happy to attempt to show that I don't see it as a foolproof answer.
The poster also couches much of her lament in terms of "primaries" which seems as though she's favoring an open marriage rather than a "non-hierarchical" poly, which seems to be what she engaged in in her youth, perhaps not on purpose.
I appreciated her pointing out things about my monogamy that I don't notice as helpful/useful/pleasant/good. The advantages of having a solid lifemate are important and perhaps I'd not given their due.
The drawbacks to her polyamory practices can just as easily happen to single monogamous people though. Divorcing can lead you to agamy also. Thus, I will not abandon my presenting the poly option to sexless spouses. It could prove to be the best option. Decades invested need not be seen as "wasted".