If you click the little magnifying glass up in the top right of the forums, and do a search for "genre," you can find three or four other threads of people expressing frustration with the Only One Genre system. You're definitely not alone in that complaint!
The difference is that if you come to the website, click on "Comics," and browse, you can sort them by genre. You can't sort by tags -- you can only see things with a tag if you do a search for that tag. Also, you can arrange comics in a certain genre by things like "most popular" or "most recently updated," but I'm not sure what determines the order of search results, so you can't sort comics in a certain tag.
(as an aside, I really really think that making it possible to sort comics by certain tags would solve the genre problem)
Overall, I agree with @lizm -- I think the best choice is to pick what genre's expectations are most likely to match your comic. For example, if you had a story about the day to day life of magical family of elves, it would probably be better to pick "Fantasy" -- the average person who would enjoy that story would never think to look through all the mundane comics in "Slice of Life" to find that story, even though that genre is probably more accurate.