Hi there. I read your first post, skimmed your others, and didn't really read the other replies so I apologize as I'm sure I'm going to repeat things that have already been said.
First off, it CAN work. My gf and her husband have been married for 7 years and together longer than that. She and I have been dating for 2.5 years. In 2.5 years of dates, sex, "I love you"s, romantic presents, etc, our relationship, as far as I can tell, has never once caused her husband to feel like he's not secure in their relationship. They were married before. They're married now. Even if she and I someday (unlikely but possible) developed a primary relationship and had a handfasting and I moved in with them and he and I were on equal footing in terms of our relationship with her -- he would still be JUST as important to her as before.
This is a tired analogy, but it's like with kids. You have one and it's the most important thing in your life, you'd die for it, you make all decisions with its best interests in mind. Then you have a second kid. And now there are two most important people in your lives, two people you'd die for, two people you build your world around supporting. And yet, if you're a good, conscientious parent, loving the second doesn't take away from the first. How does that work?? Simple -- love isn't a finite resource.
Time, of course, IS finite resource... so what about the time issue? My relationship with my gf has not once, as far as I've been able to tell, taken away time that she and her husband *needed* together, any more than her other friendships or her hobbies do. If she spends a night at a dance class, or a night out on the town with her sister, or a night out with me, how does one take her away from her husband more or less? Everyone should have a life outside their partner.
There are a number of things that make this all work. One is that he and I get along great -- we're all friends from college. So the three of us can hang out happily. This isn't the same as a date between just she and I, but it lets us spend time together and continue to build our connection and rapport, to feel like we're in a real relationship, while not taking her away from him as often. These three-person hangouts also help ensure that he and I continue to trust each other and support each other. If he ever DID start to feel insecure about my relationship with his wife (unlikely, but let's imagine), that would probably be mitigated to a great degree by hanging out with me, watching cartoons, listening to music, etc.
Communication, honesty, mutual support, respect, trust, transparency -- all of this makes it possible. Clarity is important, for me. I have no illusions that Gia is going to leave Eric for me (nor would I ever ever want that!). I trust that if I really needed her, she would be there. But if she and I were planning a date and an emergency came up with her husband or her child, we would reschedule. Now, if that were a regular occurence it would be a problem... but that would be true even if she were just a friend... I actually had a friend once who was always canceling our planned hangouts. I don't bother trying to hang out with him anymore.
All of that said, it seems like you and he have a ways to go before you're ready for poly.
So, he said one thing at the beginning of the relationship and is now saying another. Let that go, it's no use saying "that's unfair!", the situation now is what it is and you have to deal with it. BUT, especially since this was sprung on you, he has no right to rush you. You should have as much time as you need to digest this, read things, talk it out with him, etc.
His lack of transparency is a MAJOR problem. He needs to deal with this before you guys go forward. Maybe couple's counseling? Maybe a list of types of things that a) you need him to disclose immediately (ex. interest in a new person, a problem that he's having in his relationship with you, etc.), and b) a list of things that must be talked about and mutually agreed upon with you before he acts upon them (ex. going on a date with a new person, initiating sexual contact with a new person). Of course, no list can ever substitute for the right intentions, so he needs to demonstrate to you that he WANTS to honor your feelings and be forthright with you more than anything else.
There are many things that are reasonable to ask for in addition to those I began to list out above. That you get to meet his potential partners before they start dating. That he run any potential date nights by you so you can make sure it works with your schedule and even make other plans so you're not at home alone. That he spend X number of nights at home with you, so you don't feel neglected, that he and you continue to do special things together, that he only date ONE new person to start, so you all can feel out the dynamics time management, energy, etc.
There are also many things that are reasonable for him to ask of you. That you honestly tell him your problems and fears so he can address them, that you not try to force him to choose between you and a new partner once they've begun dating if at ALL possible, that you not invent emergencies to make him cancel dates or do other passive-aggressive things to keep his other relationship(s) from flourishing (not that you would, but some people do...).
These agreements/understandings/rules/boundaries/what-have-you will be unique to the two of you, of course. And, again, I favor good intentions over good lists any day. But since clarity has been a problem thus far, these sorts of agreements might be helpful.
Poly doesn't have to be, and indeed never SHOULD be, a way to avoid dealing with problems in your existing relationship(s), a way to run away or avoid commitment. In fact, if done well it can do quite the opposite. It can force you to drag any issues with communication and trust out into the open to be dealt with, and can help you clarify why you're important to each other (if "exclusivity" is the only answer, well, doesn't sound like much of a relationship to me!).
A time when you're having other problems in your relationship (I believe I saw the words "bad weird" above) is NOT the time to open up to poly. Deal with those problems first. Be as strong as you can together. THEN open up. That is like Rule #1, and it's as much for the benefit of any new people as for you guys... anyone he might date deserves to enter a healthy situation, not a powder-keg of drama.
Phew! This DOES sound like a lot of work and extra complication! Why do it? Well, for one, if he really does need it, then he will grow resentful and depressed in the long-term without it. Personally, I find the extra support and love that multiple relationships provide to be immensely useful and awesome. We each have an expanded network of support, even Davis, my mono bf who's not involved with the other two beyond the level of acquaintance -- if he badly needed help I know they would come, for my sake.
Good luck!!!! Maybe your bf should read this thread?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.