When you go into business with friends, you are already jeopardizing your friendship. That particular ship has sailed.
You now have an option to (1) do what's right for you, and (2) potentially have some friends at the end of it. BTW it's not guaranteed that you can have either of these, let alone both.
If you are convinced that you want to leave, I think the best way to phrase it is "I no longer believe that the direction we're headed is the right one. You deserve a co-founder who is all in and believes in the mission. Even though I know this will be painful for everyone, the best path for the rest of you is if I leave now." Expect to give up most or all of your equity position, and maybe stay working a little longer than you'd like to get things wrapped up and in a good place to hand off. Also expect that this won't work and everyone will still be mad at you, but you'll learn a lot and your conscience will be clear. Maybe someday your friends will stop being upset and will realize that you were right, and you can be friends again. No guarantees though.
This is 100% true if you did not do your due diligence before you all committed to whatever you are building. One of the best ideas is to hire someone like me who has failed at building something and helped other people try and iron out some of the unanswered questions. Someone else already mentioned this. He is likely just as capable. It may take a fresh set of eyes and a fresh outside voice. I can point you to other people as well.
The actual potential market size may not matter. It is never a great idea to build a business from the top down. Instead, build from the bottom up by seeing if you can sell your product to whoever you think is the customer. The true test is right there. If you cannot get all of the stakeholders to the table due to needed partnerships, it provides another precautionary red flag.
Maybe there's not really a legal way to protect any of your equity assets when they aren't worth anything. I'd imagine any future investor interest would be scared off by the disclosure that there's an uninvolved past founder with "25% equity" on the cap table. Rather than worry about potential last minute negotiations with the ex-founder after an investor expresses interest, I'd imagine the remaining partners could potentially found a new group and dissolve the current group by "selling" it and all assets to the new group for $1 and mail the ex-founder $0.25 or something.
This would likely be moderately illegal if done at a stage that the equity is demonstrably worth something (e.g. Eduardo Saverin's FB situation) but I don't think it would be much of an civil/legal issue before the startup had any demonstrable financial value. I could be wrong. And people will diverge widely on how they see the moral/ethical implications of this.
I think that if they are really going to turn what we currently have into a company on facebook scale, then this value will have to be created after I leave. Right now, the startup is certainly worth almost nothing.
You now have an option to (1) do what's right for you, and (2) potentially have some friends at the end of it. BTW it's not guaranteed that you can have either of these, let alone both.
If you are convinced that you want to leave, I think the best way to phrase it is "I no longer believe that the direction we're headed is the right one. You deserve a co-founder who is all in and believes in the mission. Even though I know this will be painful for everyone, the best path for the rest of you is if I leave now." Expect to give up most or all of your equity position, and maybe stay working a little longer than you'd like to get things wrapped up and in a good place to hand off. Also expect that this won't work and everyone will still be mad at you, but you'll learn a lot and your conscience will be clear. Maybe someday your friends will stop being upset and will realize that you were right, and you can be friends again. No guarantees though.
Best of luck to you.