You don't have to take up the "poly" label if you don't want to.
You're still welcome to be in the clubhouse & consider yourself a member just as much as you please.
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Despite often being a very emotional guy, I'm known for being a bit hyperrational, too, so take the following in that context, 'kay?
It sounds as though you & your husband are in a difficult situation -- not perilous, but definitely troublesome.
And I'd say that you having a steady "outside" relationship is playing a major role in
keeping your mariage together.
He can get pulled away from you yet be certain he'll at least still have a home to return to, & that you're not likely to suddenly replace him.
You can be certain that the man you loved enough to marry will return to you, & in the meantime that you won't drive yourself crazy from loneliness & possibly leap into an ill-advised relationship.
Guessing a little, it seems as though your boyfriend has two women who love him.
It's far from ideal, sure, but from what you've said thus far it appears
manageable as you build toward something less overall stressful.
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Are there long-separation stresses? Yep -- as Kevin said, military deployment does a lot of this.
My wife was never shipped out, but at one point she spent half a year in Monterey, stopped off in Minneapolis for a weekend, then did four+ months at the Pentagon. Before she went west, though, I warned her that if she got any ideas about not coming home, she'd better know it couldn't be retracted. Naturally, she took it lightly, said I was getting panicky, but agreed to abandon all claims on our house & property if she left me.
Actually, I wasn't any more worried than usual. I knew her very well by then.
She did a lot, saw a lot, learned a lot, & met all sorts of new people. Months after her return, she finally admitted that she came back intending to file for divorce; our contract made her stop & think, though. It was almost like a sort of
negative NRE where everything & everyone & everywhere else were all wonderful, & all she could focus on was how badly I looked in comparison.
I loved her, & I worried about her, & I knew it was impossible to argue with. I was ready to take the property, accept the divorce, & split the cost of legal mediation.
Meantime, I encouraged her to keep in touch with her exciting new friends & lovers. It took more than a month for her to remember what a great team we were, & by then she was already losing interest in chasing off after new experiences.