i think the real issue here, from your post, IF I understand things correctly, is about rules and you feelign comfortable acting on your wants/needs and having hubby be ok with it.
I think GG is right, it's time to re-evaluate your rules.
Leave M out of it, and re-negotiate your parameters, then, after that is done, you can safely pursue M with an established set of rules that maybe more conducive.
When you do that, you have to express your desires and stand up for what you want.
-- For the pursuit of happiness, not the sit around and wait for happiness -Barney (NPH) How I Met Your Mother
Jen - bi female
John (Juntas)- husband
M - John's girlfriend
R - My whatever, he is beyond labels at this point
currently personally closed to more, well maybe not to M