i think it's a freud thing, you know, that psychosexual development?
1. oral phase - too much or too little oral stimulation, like being over or underfed or sucking one's thumb, in the first year of life leads to oral fixations. babies "sex drives" are relieved by oral pleasures. so it leads to behaviours like smoking and nail biting and such. sexually, it's why some of us like oral sex.
2. anal phase - too much or too little parental concern over cleanliness during potty training (around age 2 or so). in this stage the toddler's "sex drive" is satisfied by being able to control (or not having to worry about controlling) one's bowel movements. a crisis in this phase can lead to what we call being "anal-retentive" and being obsessed with cleanliness. But on the other end of this spectrum, one could argue that a crisis in this phase could possibly link anal pleasure and bowel movements to sex. so sexually this could lead to a more normal response, anal sex, or the less common, involving defacation and fecal matter in sex, essentially, shitting on one another.
3. phallic phase - the age where we usually first diddle ourselves and sexual orientation is affected and we love our mommies and daddies sexually (this is where Freud loses me)
4. latency phase - we dont care about sex much here...
5. genital phase - OMG PUBERTY YAY!

and after all this and puberties all started and were all sex crazy, all these stupid developmental things show up in both private (in ze bedroom) and public life

so yea, that's prolly how freud would explain it. i mean, i still don't quite understand why you'd want to shit on your lover... XP