I have a partner who is gone for a couple of months at a time. We have not ever held each other to a completely monogamous relationship, because we both get lonely. However, there is no replacement for each other in our lives. Since then, I have met another guy, who is wonderful. We are now also in a relationship, with everyone aware of what's going on. The reassurance is that I care very much for both, but my partner will always be my mate. So far, it seems to be working, but there are a few questions and concerns. The rule between us all is that we are completely honest, and not threatened by the other persons. None of us have any other interests in pursuing any other relationships for the time being.
My partner and the other guy do know each other, and are friendly, but haven't had the opportunity to be intimate at any level. I'm hoping that will happen.
I think it's risky in some ways. We have rules about safe sex, making sure no one brings home a bug, etc. But I have to say that this relationship with these two people has brought a lot of satisfaction and happiness to me, and so far, satisfying to all parties. However, things have a tendency to change, and as long as we are honest, and can work through our feelings in a mature way, I think we can continue. We just take things a day at a time.
Incidentally, I never dreamed I would be in such a relationship. At times it does feel a little unnerving, but mostly it provides a sense of completeness.
I'm not sure I recommend it either. I can see that such a relationship is filled with complications, but so far, it seems to be working. It's definitely not for everyone, if you intend to act responsibly about feelings, etc.