Responses
You know what you’re talking about. Tampons and sometimes cotton balls wrapped in cheesecloth are used a lot. It literally doesn't take much to get tipsy/drunk using this method. Just gotta know what you’re doing. That’s all I gotta say.
Kaktus probably had a shitty experience or knows someone that did but was inexperienced in the whole thing so now they’re salty about it.

Right, so: alcohol enemas aka butt chugging. It can be extremely dangerous, fatal even, when done improperly. Butt chugging often is a quick way to get drunk, since your blood stream absorbs the alcohol directly instead of having it completely processed by your digestive system. As such, the unfiltered/unprocessed alcohol reaches the liver and overwhelms it, which can lead to alcohol poisoning and death. And since it comes in via your rectum, there's no defense mechanism (vomiting) to help get the toxin out.
Now to the specifics in the scene: usually, a tampon soaked in alcohol or a funnel is used, not just directly inserting directly from the bottle. (That in itself is another danger btw, be careful of glass toys in your butt-- I have heard too many horror stories of glass dildos shattering whilst inside an orifice.) Champagne is a low proof alcohol, so it's much safer in this case although that doesn't detract from the dangers stated above. The bit about feeling the bubbles is quite true though! It's quite uncomfortable though, lol