This is kind of off-topic for this thread (and maybe the mods could split this to a seperate topic), so...
@QM: I would advise against doing something out of spite or to "test" your partner - down that road lies all sorts of ruin.
But if the two of you have agreed that your relationship will allow others in and if you want to meet up with this person and think you would enjoy spending some time with someone, then why not?
Have the two of you talked about boundaries at all? Things like safer sex expectations, and things like that? (and I don't just mean for you, but for him too)
The gut feeling I have from reading your posts is that things are kind of spiralling out of control in your relationship right now, that you don't know where you stand and that maybe just adding more chaos into the mix won't actually solve anything.
Mono/poly can work out if that is what each partner wants. If it is dictated by one of the partners ("I can have more than one, but you're not allowed to") then that tends to lead to bad feelings of unfairness.
I believe that part of the growing process is very much understanding what you do and don't want for yourself. The next step is to communicate it to the partner that you love and have made a commitment to. This is hard when worlds suddenly start opening up and you don't really know what you want, but then the process should be one that the couple take together, rather at odds with each other.
For me, I tend to like things to go slower, making sure that everyone is OK, before anything else is undertaken.
"Listen, or your tongue will make you deaf." - Native American Proverb