Well J is off limits because I met her so closely (in terms of chronology) to when I met S. S is not comfortable with this and I've always said she is my main priority... I always saw these issues as being to do with insecurities and things that might one day be possible to work through but I know following our argument yesterday this is a firm boundary that won't change.
More generally speaking S would have a problem with me dating anyone that mixes (or might mix) in the same social circles as her. So that puts anyone on the kink scene or poly scene as off limits too. These are the main groups of people I would be looking to date, obviously only if it felt right. Alternatively I would be happy dating someone who I found who was curious about these points... but if she started dating me as well she would probably become a part of that scene quickly, unless she specifically wanted to be closeted about it. Now J is in the kink scene and is in an open relationship, she's not tried poly relationships before but would be open to the idea. Plus it felt like we had that connection, which for me is really rare.
The only options that all that leaves me is something long distance. My last relationship broke down about 3 weeks ago and the primary reason for this was distance. For this reason I would not want to intentionally seek out something long distance out of choice, if it happens and it feels right then who cares about distance... but I don't want to specifically date people outside of London out of choice.
The only other option this leaves me is being mono, I'm totally fine with this so long as I'm allowed to get in that mono head space. I really think given that LGBT can be thought of as a spectrum with people being heteroflexible/bi/homoflexible etc, then poly/mono should be thought of as a similar spectrum. I sit somewhere in the middle of that spectrum and am equally comfortable and have my life equally fulfilled with following either lifestyle so long as it fits the relationship(s) I am in. I don't feel anything to be missing from my life by identifying as mono, but I'm having a hard job convincing S of this. I think it's because of this that she's really beating herself up and blaming herself for forcing me to be monogamous.
The flip side is that if I'm identifying as poly and that is ok with my partner(s) then I feel restricted by having everyone off limits. It's all about the label I'm using and the way I'm adapting myself to that label and set of boundaries, I've been referring to this as that kind of self identification headspace and how you see yourself is key to how you're going to seek to behave. But I can certainly be flexible in how I identify.
I love her to bits and am terrified that I could lose her unless it's discussed properly.
I went to a poly meetup group in London tonight and some of the people there were really great. If S had been able to hear some of the things that they were discussing and if she had been able to challenge them and ask her own questions then it might have helped challenge her own thoughts and reasoning.