I've never really had experience turning things from a monogamous relationship to something more. Every time I tried to introduce other girls or people into the relationship after a few meets, the girl wasn't up for it because she liked me too much by then. So, to lay out a counterpoint to Solong, I think guys should make it intimately apparent from the beginning that this is a different kind of relationship - maybe even bring another girl on the first date, or make your first date going out and talking to other girls / guys.
I have a knack for getting into situations with two girls and sleeping with them both at once, and a couple times this has turned into a continuous situation.
I've turned a lot of my relationships into open, polyamorous, or swinging ones since 30 and because this is an integral part of swinging. Being a bit older as a guy, might help when dealing with women in this context (maturity or relationship experience). And is something very different than what even the average PUA deals with. There is the concept of "overwhelming" a woman or pulling her too quickly out of her comfort zone, and it especially appears to be the case with Japanese more so than Western women. "You want to go as fast as the woman can handle or is comfortable with."
By throwing everything immediately on the table, many women can take it the wrong way. They just met you, so tend towards fear, suspicion, or paranoia. That is too quickly assuming the guy is a dangerous deviant or pervert, or she doesn't want to give the impression that she is a "slut" or "easy" girl.
And women can be tricky about this too, as pretend she is open-minded or totally going along with what you are saying, when she is NOT or totally and negatively change her mind a short while later. Very common in Japan (honne and tatemae). Agree with everything you say today, then tell you she's not with it tomorrow or days later.
Consequently, it is better to actually establish a sexual relationship and connection with her first.
You can indirectly talk to her about the concept of open-relationships, swinging, or polyamory but arguably better not to put any pressure on her to decide too early or right away. More like a long general fun topic discussion. Often better to explain it as something your friend or 3rd party has done or is considering, to separate it from yourself, before you had sex.
The "kind of" main exception is arguably pulling women into 1st night sex orgy situations. Like you and your pal, bring her and a girlfriend back to your place/hotel, and you have a 4 way sex orgy. Another is bringing her to a happening bar... Here, you established an IMMEDIATE sex relationship, so HONESTY about anything sexual is usually easier to discuss. She can't claim she's "mother Teresa" or "not that kind of girl".
So:
1) Talk around it.
Talk about open-relationships, polyamory, swinging etc.... But don't put any pressure on her to decide immediately. Just generalized discussion at first. Gauge her reactions.
2) Establish a sexual relationship
All the talk about open-relationships and polyamory means ZERO if she runs away or she friend-zones you and will do nothing with you.
3) Go as fast as she can handle
Women have different speeds. Figure out her comfort zone, then proceed at that speed.
With some women, we were fine with an open-relationship and swinging from day 1. With other women, took 2 to 3 weeks (depends how often you are meeting too).
4) Do address it at somwhat at the BEGINNING of the relationship.
Don't wait months or years (have seen this) after being in a seemingly monogamous relationship, to suddenly try to convert it into a polyamorous or open one. Because that's usually massive emotional drama.
Here, I'm in agreement with you, about doing it early. Just a matter of exactly when and how early.
5) Undeclared Open-Relationships
Women and men can play a game, where they call each other boyfriend or girlfriend, but don't see each other often. Very common in Japan.
If you are seeing him or her 1x a week, only 3x to 4x a month, then what the hell are they doing all the other time? Many people are "cheating" and have 2 or 3 other sex friends or boyfriend/girlfriends.
This is important as far as polyamory or open-relationships go, because some people prefer to lie, hide, be selfish, and cheat as oppose to honesty. You have to keep this in mind. She or he may say that they are totally against such open or polyamorous relationships, but in actuality you ARE in an undeclared and unmentioned 3-way or 4-way relationship but don't know it. They just prefer not to discuss it and do it behind your back.
My experience is non-existent for finding bisexual guys though, sorry!
A guy finding bi-sexual women is admittedly hard. But there are various ways.
1) Bi-sexual female friend story
One way that I pull it out of some women is tell them stories about bi-sexual female friends of mine or my pals. I distance the story from myself or her, as this makes most women more comfortable.
I then ask her opinion about how she would react in various bi-sexual situations that my friend found herself in. Based on her verbal, facial, and body language reactions you can figure some out or at least she is comfortable with it.
The REAL test, is to put her in a female bi-sexual situation, like a swinger party/sex orgy, swinger club, or 3-way sex. Words and actions can sometimes be very different.
2) Use bi-sexual women to find other bi-sexual women.
Women can have strong lesbian-radar. I've had bi-sexual or lesbian women approach girlfriends of mine. I'm sitting there and literally watching them flirting and trying to get with my lady. Even at times where my girl has no clue what's going on, but I do, as have seen it before. I use those opportunities to bring us to our apartment or a hotel. [emoji48]
Even better when my girlfriend is openly and comfortable being bi-sexual. She goes get women. Such an interesting thing to watch. Those are diamonds, and hard to come by. Sometimes just sheer luck or requires going through a lot of women, and the guy being very mindful to look for that type of woman.