Darou said:
closest to this particular image would be adultery,

Eehh.. while that is probably the closest tag, adultery does not feel like an accurate tag for this situation.

It's technically accurate, but very much implies disloyalty and faithlessness, which I just don't believe applies to this.

and more specificly swinging
(https://en.m.wikipedia.org/wiki/Swinging_(sexual_practice)
(snip... resnip..)
PS: swinging might require a disambiguation...
-object(ex. sword)
-activity (ex. communal sex)
-movement (ex. hanging seat)...

Yeaaaah...

there is also partner_swap but that seems to require at least 2 pairs/duos and isn't compatible with triangular relationships.

agreed.

There's also cuckold however, that implies more of a ... "I am TAKING your partner" or "I TOOK your partner" with overtones of cheating and humiliation.

Man, there's a whole jungle of sex-with-people-who-aren't-your-partner tags.

Hm...

presenting_partner - 251 posts - This tag indicates that where two or more characters are in view, one is taking action to put the other in a presenting position. This can be done in a consensual manner, as a shared act of exhibitionism, or in a non-consensual, forced manner as an act of domination and submission/slavery. In the latter, the situation may be in a private or semi-private context for forced prostitution, or as an act in a bdsm scenario. Depending on the context, the intent of the dominant partner may be a type of reverse-voyeurism referred to as candaulism.

Dang, that's a nice wiki. Not really applicable here, but it'd be a good place to look for potentially applicable posts (for example, post #1477830 might count, depending on how this tag is defined)

There doesn't seem to be anything else I can see.. hmm.

Sharing_partner? partner_sharing?