Alcohol gets on the breath in the lungs, not the short little bit of shared plumbing between the digestive and respiratory systems.
I doubt it makes much difference how it gets into the bloodstream.

The main problem is probably that the colon's function is to extract water from the digestive goo. Alcohol probably moves through in about the same way, maybe a little faster than water. The rest of the digestive system is more geared towards the nutrients. So it probably isn't so much a matter of filtration as just plain transfer rate.

If the alcohol poisoning doesn't get you, the process is pretty likely to dissolve toxins that the body was inches away from getting rid of and carry them to the bloodstream. The digestive system generates some really nasty chemicals.

All in all it's just a really bad idea.

And that's when it involves the colon. The other version is probably less lethal, because those mucous membranes are not all about transferring fluids. It might not even be a particularly effective way to ingest alcohol. But the reproductive system, dang, it's delicate. And when the chemistry isn't right, so open to infections.

It's just a booze-buzz. And we've all got a perfectly good booze-port. "Bottoms Up" should refer to glass not glutes.