eatmice had posted this saying their experience of this in the past. There really is no reason to get upset at someone for asking if others do this because of something that happened in the past (that ended up fine for them). Oldfishlady does it, so do some other breeders on the forum here.. eatmice was just asking if others had success in this too.. not asking if they should do it again, etc.
A lot of people who don't research in depth don't know they can't do this and end up having this set up and with the fish not harming each other their whole lives. The longer they are together, the older the fish, the more calm they are and the likelihood of problems arising after they were together for 1.5 yrs is low. So some people never are aware of the problems that could of happened at the beginning.
No, it's not recommended as it could easily turn south if the person isn't watching closely, or doesn't know their betta's "personality". But it does happen regularly to many people - they will intentionally place them together and have no trouble. Heck, many of my pairs that are virgins I keep them in the same tank for a long time together while they "figure out what to do", and they either just end up doing their own thing or they become friends and will literally sleep side by side in the plants, etc. Some of mine live just fine together for a long time in anything from 1.5g - 3g with me.
If one is overly aggressive then no, I wouldn't keep them together, but I have very "gentle" ones that have no issue living with 1-2 more bettas regardless of sex. Many breeders/people will keep 2-3 females in 3-5 gallon tanks for years and have no trouble with aggression at any point.
It all depends on the betta itself, if it's one that is a bit more aggressive then it won't work. If it's more docile then it may work with the right pairing/s. People who attempt this should be very vigilant, and know the signs to watch for, and to know when to separate before it goes too far. I only stand a very small amount of chasing before I start to watch very very closely.. I have learned what to look out for - length of time each chase lasts, how determined the one is to catch and the other is to get away, body language, mouth position, etc. And anyone else who keeps their fish together in unisex tanks successfully does the same thing.
I only would do male/females together - I wouldn't attempt two males together personally. My males who pay no mind to the females when not in breeding mode will still try to attack any male it sees.
No, it's not recommended because many people don't know what to look for when selecting fish to keep together in the same tank. Or they create an environment more for show than for "natural", etc. No, it's not recommended because most people don't even know the proper way to care for these fish and shouldn't try something that has potential to harm one or more fish. Under certain circumstances I would be more than happy to help someone set up a community that includes both a male and female/s, but generally I say no.
You would be surprised what actually goes on, what experienced breeders/keepers (and I'm talking decades, etc) actually do and how they keep these fish that if they were to say anything on these forums they would be bashed and yelled at. I have talked to some of the really experienced people here who have told me just that.. they say one thing but do another because of how many people/new owners think. It's not a knock on people who are helping, or on anyone in particular - but keeping a betta for a year or two doesn't equal knowledge. It means you know your fish, and it means you know the basics. But as far as how to keep/care/breed/feed you will be surprised at how it differs between people with a few pets and people who have hundreds, thousands.. and people all over the world.
So show a little peace.. don't judge.. recommend not scold.. a question was brought up, not a debate. :) We all love these fish, if we did't we wouldn't be here.
Breeder so I have too many to count and too many named to list here!