Wow...lots of negativity. I understand some of it, but maybe it's due to only giving a very few facts.
People tend to be negative about things when they have either seen or lived patterns and try to project those feelings on people who are asking questions.
Negativity and "reality" can run very close together. But people who have traditionally seen lots of negativity only sometimes forget, anything is possible.
Poly is a small subsection of the non-monogamous world. Non-monogamy is a small part of the overall relationship world. You have found something, you can see how it might work, but its good to know the relationship risks. (like STD's but with relationsips)
All these people saying you shouldn't do it, should instead simply be telling you the risks and letting you make an education decision. We are all adults, if I want to jump off a cliff I damn well can, but I can be both protected and enjoy the ride, knowing shit may hit the fan. Sometimes you just have to weed through the negative crap and build your own risk assessment.
Good luck, sounds like you have some fun NRE, for the record sharing the bed with a woman does not mean you need to be involved with the woman. Men do it all the time, I find it funny so many people are clueless about the mechanics behind 3 straights. (if the hubby is expecting some lesbian love, thats a whole other bucket)..
Also, things evolve. They really do. If this is what you want, and you understand the risks, it is POSSIBLE that she will adapt and evolve and feel safer with your involvement in the relationship. If thats something you, and he... and even her are willing to work towards and re-evaluate, then go to town and enjoy the love.
Naysayers are usually the loudest and most annoying, just take what online people say with a grain of salt. Remember people want to project what they see most.. I usually just view it as comedy. In some cases, they even forgot how they got started in this interesting world. ..
Welcome to poly and the forums..