I think it's really important to realize as people that what we think are boundaries and what are actually boundaries are important. My brain doesn't always work well so I'll clarify.
Okay, so both hubby and boyfriend are stating they are uncomfortable with share time, living together or anything THAT close. Yet both say the closest they would be comfortable with is say, sharing a duplex. Hubby, me and kids on one side, boyfriend on the other. So in their minds, this would be comfortable for them. Yet, I don't get super excited and start looking for duplexes. Why? Because the reality might be different. Things need to go in stages.
Hubby calls it emotional yoga. He hears the phone and knows it's boyfriend. At first, uncomfortable and leaves the room, not sure if I need privacy or if he wants to hear anything. The next time the phone rings, less uncomfortable. Like stretching, he know doesn't care and sometimes won't even ask if it's boyfriend or a work friend because he just honestly doesnt' care!
So, you are uncomfortable now with this level of interaction. So don't do it! If you do want to try and build up to it, that's fine, that's your choice. Just try to remember uncomfortable is not a no, it might just be emotional yoga, if it's TOO uncomfortable or painful, yes, pull back, don't want to pull an emotional muscle.
Maybe it's a level of interaction you will NEVER want, and that's fine too. It's awesome that you are willing to entertain the idea. So discuss it, discuss what level of interaction you are willing to start with, I like the signal idea as well, kind of like tapping out when your energy is spent. If afterwards you relax and can look back and say, "Okay, well I could probably give that another go at another time and do better." Then great! You are stretching! If you look back and go, "Okay I never ever ever ever want to do that again!" Then fine, that's the point where stretching can be dangerous and hurtful and you stop.
Everyone is flexible to a different degree.