I think it can be challenging to find people you can play with who are open and honest. But it must not be impossible, or we wouldn't all be here!
How important in your life are these guys you are sexting -- ? If they are, maybe you could talk non-sexy with them sometime, about what's going on in their relationships. Who knows, maybe you could help them improve their love lives, in ways they never even imagined! Poly isn't just about sex -- why not bring some love and care into it? The fact that you are even asking about the ethics of your situation leads me to believe you have a good heart. Glad you're here.