Wow, great responses! This forum is wonderful; thank you for your observations and wisdom - was feeling "stuck" on this and feel much clearer even just from having written the questions down, as well as from all of your responses.
Finding balance is a part of poly that both E and I is definitely something that we have struggled with coming out of the gate. "More people, more problems" is the motto, I believe.
I suppose watching someone else have five (now six) partners, and still be ready to date other people has really raised the question of, "How much time and energy do you have for anybody if you are giving it away to everybody?" Two of her partners also carry STIs, which is another scary factor to me.
AnnabelMore: Your post went straight to my heart. Yes. This.
It has definitely made us realize that we don't want to try and invest in so many people again in the future. Elemental and I had some long talks this morning about the importance of the sexual and emotional health of our relationship in the future, and how we don't want to put either at risk through irresponsible/unconscious expansion of our love.
Clarity is invaluable, as is this community. THANK YOU!