「Horizontal rod motion on the inside」suggests one party HOLDING STILL, NOT MOVING, NOT CHANGING POSITIONS.
「Array of persons」「inside widely spread legs」and indicates a SOLID STANCE, IMMOBILE, NOT ABLE TO MOVE. A position that would cause the person with legs spread to become IMMOBILE (due to being tied and bound tightly?) as the other persons inside those legs do their work. THIRD in a series of similar of events, as perhaps in the sequence: a) foreplay, b) coitus until climax, then finally c) refractory resting and IMMOBILITY…or from the perspective of the female, the third phase in which she finally takes charge and pleasures herself (see 柄). Note that the elements as「one wife」simply skips to NO MOVEMENT, IMMOBILE and avoids the bother of a) and b), with condition c) prevalent after the honeymoon buzz wears off in many monogamous marriages.
Later usages have meanings of CONSTIPATION as in「widely spread legs and remaining immobile while constipated」「array, line-up of erections taut within spread legs」or more to the point as IMMOBILE BOWELS, NO BOWEL MOVEMENT. Also a POSTURE FOR DEFECATING, and most folks would agree that hopping around while defecating is at the least poor etiquette, let alone unwise since the hopper might well hop on his or her own feces, slip uncontrollably, and sprawl backward.
Yet, this glyph was used in the ancient zodiac and astrological charts to mean our flashing, bright, shining Sun that was worshipped by some, still is worshipped by some others, perhaps depicting the「immobile, innermost part of the wide heavens above」relatively speaking, and within our own solar system. This glyph was used to refer to 3RD in a series and the third shift of the night watch (11 p.m. to 1 a.m.) when the Sun would be aligned on the opposite side of Earth relative to the subject's speaker or writer,「deep inside」the middle of the night, or midnight.
Scholars say that this glyph refers to an altar for performing meat sacrifices, perhaps because cave folks worshipped the Sun, or because altars also are relatively IMMOBILE, but not too many folks worshipped the Sun around midnight, other than those not wishing to be burned by it…perhaps then, as now, melanoma was a concern.
The so-called「ten heavenly stems」(十天干) all have something to do with a male erection or the application thereof, which is exactly why they are called screwing, full and heavenly and stems: 1 甲, 2 乙, 3 丙, 4 丁, 5 戊, 6 己, 7 庚, 8 辛, 9 壬, 10 癸, and this one seems to refer to a gang rape of one or more flashing orifices.

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: third of 10 heavenly stems 十天干; third in order; letter "C" or roman "III" in list "A, B, C", or "I, II, III" etc; propyl
Japanese EDRDG: third class, 3rd in a sequence, 3rd calendar sign
Unihan extended: third; 3rd heavenly stem