The Physiology of
When we drink alcohol (or take medications) by mouth, and they are absorbed into our bloodstream, they are taken by a network of veins called the portal venous system directly to our liver and usually at least partially metabolized. This is called "first-pass effect." The veins of the stomach, small intestine, and most of large intestine drain via the portal venous system.
However, there are two small veins at the very end of the rectum (called the middle and inferior rectal veins) that drain _directly_ into the veins of the systemic circulatory system -- thus, anything absorbed via this route goes directly into the main circulation without being subjected to first-pass effect.
I'm not sure that anything absorbed via rectum is in fact absorbed significantly more quickly (I might be wrong about this) but something aborbed via the last few inches of the rectum -- remember, most of the large intestine is drained via the portal venous system -- does reach the systemic circulation without having been subjected to first-pass effect and is thus in a more unmetabolized, and thus often more potent, state.
It's worth remembering that alcohol is indeed something of a poison and if you take too much too quickly it can be directly fatal. This typically occurs when one's blood alcohol reaches about the 0.4% level -- with 0.1% or 0.08% usually considered the upper limit of alcohol level compatible with the ability to drive without being significantly intoxicated.
Alcohol taken by mouth has something of a very primitive protective mechanism in that the more intoxicated one becomes the harder it is to continue to drink, and this makes it difficult to ingest a fatal amount if one drinks relatively slowly. (Alcohol is absorbed more rapidly by a relatively empty stomach and more slowly by a relatively full stomach. It should be noted that various cheeses have something of a "coating action" on the lining of the stomach and thus are particularly effective in slowing the rate of alcohol absorption.) Alcohol given by enema obviously lacks this "safeguard" and can even be given to an unconscious person.
Most of the case reports of fatalities I have heard of that were related directly to the toxic effects of alcohol -- as opposed to intoxication-induced injuries, aspriation of vomit, etc. -- were due to binge drinking where the person drank a large amount all at once by mouth (I once saw a half-gallon-sized beer mug with "I bet you can't" printed on it; personally, I found that a rather chilling sight, and maybe a future wrongful-death lawsuit looking for a place to happen) or was given a large amount of alcohol by enema. Fraternity initiations account for a large percentage of such cases.
As an entirely separate issue, it's worth keeping in mind that a "substantially" intoxicated person is considered unable to meaningfully consent to sexual activity. It's very well established in law that getting someone too intoxicated to understand what is happening to them and then having sex with them is rape. (How the authorities would regard someone's giving "prior consent" to becoming intoxicated and then participating in sex and/or BDSM, I dunno.)
If one decided to experiment with an alcohol enema, it would be wise to keep in mind that it could go into the system of the recipient in a more potent state (and that once it's in it cannot be removed except by being metabolized) and adjust both the dosage and the rate of administration accordingly.