It’s true that the relationships would probably change if you told the people about each other. And I’m not surprised you want to keep things the way they are, since you enjoy it. But do keep in mind that
(1) Your behavior is unethical by many people’s systems of ethics. What is your system of ethics based on? How do you justify your behavior in your system?
(2) At some point one or both of your girlfriends will probably find out, and the relationships will change anyway. But if they find out without your telling them, they may have a stronger sense of betrayal and that may make it worse for you and them.
I’m sure you feel that you care about them.
But to me, caring about someone is not just a feeling. It also means doing your best not to hurt the person.
In this case, you are setting your girlfriends up for a very big disappointment down the road, when they discover you’ve been hiding the existence of other relationships. I don’t think that’s very caring at all. I think it’s downright mean and nasty.
When you say you care about them, don’t you really mean that you enjoy being around them? But you aren’t really looking out for their long-term interests, or even your own. You’re only looking out for your short-term interests — you don’t want to have to change the relationships. You prefer to hide your head in the sand and wait until the situation blows up and the relationships change on their own, possibly more painfully.
You have two ethical choices, the way I see it:
1. End your relationship with one or both of them.
2. Tell them both you’re involved with someone else and you don’t intend to give up either relationship. Then work with them on whatever feelings might come up.
Most people who do polyamory believe that it is about having multiple relationships *openly and honestly*, not simply about having multiple relationships. Lots of people cheat.