The etiquette question only comes in when you are dealing with a "must invite social units" situation -- like invitations to weddings, dinner parties, etc. -- not informal socializing (including visits).
In the informal situations, I think there can't really be a rule, because there are times when you wouldn't even have to invite spouses.
The question about formal invitations, though, is really interesting. And here is where I think it's important to remember that the etiquette rule is that you have to invite complete "social units" -- not that you must invite all "significant others." Whether people will be offended if their significant others are excluded is another matter -- I'm just looking at the minimum etiquette requirement here.
Social units include married and engaged couples. That's clear. Gotta invite them together to things like weddings (not single-sex things like bachelor parties or group-specific events like book clubs or office parties).
And of course many authorities include other relationships as "social units," too. Probably most compelling is in jurisdictions that don't recognize same-sex relationships, same-sex couples who get as close to "married" as possible for social purposes. I think most authorities would call that a must-invite social unit situation.
In my world, there are also couples, usually older and widowed, who don't marry or cohabit for a variety of legal and financial reasons, but are very definitely social units for social purposes. The benchmark I use there is not whether they always socialize together, but whether they always co-host together themselves (social events, holidays, etc.). Once someone co-hosts their SO's daughter's wedding, yeah, I see a social unit there.
Then it starts to get tricky, even when it doesn't sound like it would in the abstract. Take cohabiting couples: most of us would invite both of them anyway just as a courtesy matter, but even that is not always clear as a must-invite situation -- sometimes people are just roommates, or friends who are getting to be more than friends, or there is another roommate with a couple, with whom the hosts are also acquainted, for example. And you may have a very committed couple who don't cohabit who seem more like a permanent couple than a new cohabiting couple, so then you feel weird using that as a benchmark.
"Inviting all SOs" sounds easy in the abstract, but it can lead to problems when you have other people on your list who are in very committed relationships but don't live together, because it can be hard to draw the line on just which relationships "qualify" and it is horrible to be deciding who is "serious" or "committed" enough and who isn't. Hosts end up either inviting all SOs (and perhaps even feeling that they must then also invite singles to bring dates) or none (which offends many) or inviting some and not others (which also offends).
Speaking for myself, I look at permanence (which I think is the idea behind the whole "social unit" principle anyway), not seriousness. That both sticks to the reason for the SU rule and saves me from weighing or investigating (as in the case of a geographically distant cousin I'm inviting to a wedding who has a boy/girlfriend that I really don't know anything about) how serious other people's relationships are.
So it's hard enough when you are only dealing with groups of two. When you get to poly situations, I think it's helpful to fall back on the "requirement is social unit, not significant other" rule. In the OP's example -- and I am assuming we have two relationships with an overlapping person, Woman 1, not a single three-member relationship -- the man is their friend; they only include Woman 1 because she is a SU with him. So I think they wouldn't be required to invite Woman 2. They still can, of course, if they choose to do so.
But in a situation in which three people see themselves in a single, permanent relationship, and hold themselves out as such socially, I think I would consider them a social unit that must all be invited.
Four? Five? What do you do when you get to twenty?!