Philosophically, either you are cheating (guilt by enabling), or by opening up your relationship with your husband, you're in the clear (individual limitation of accountability). If you need to have a moral answer, pick one.
I prefer to think in practical terms. What's the likelihood of being found out? What are the likely scenarios if found out? Does the sneaking add value? Would not sneaking add value? What are the motivations?
If we set the moral question aside, and you're not likely to get caught, do your activities matter? Your marriage is fine, but are you destabilizing their relationships? If so, I'd suggest that they need to either fix things or move on. Being a Band-Aid for their relationship problems is bad voodoo. I would suggest that sexting means you're likely running a much higher risk of discovery. If the partner just happens (incidentally or intentionally) to check his phone, there maybe be all sorts of uncomfortable questions.
So what happens if the others find out? The options are basically end the relationship, open the relationship, adjust, or no effect. The stereotypical would be ending or harming the relationship. Thus, you could be contributing to some future distrust in relationships. Or you could be encouraging some people to open themselves up. You have mire data than we do because each case is different.
Perhaps the naughty factor gets you excited. Many responses have cautioned against being the other woman, but some women like that role. If you need the clandestine to be fulfilled, then our moral commentary is unnecessary.
Or you might feel better with everything above board. That fact you posed the question implies you have some internal struggle. Even established poly folks can have moments of doubt, but I find that being entirely open is significantly better than hiding.
You mentioned hormones in your post but talk about staying in touch after moving which implies to me more than hormones at work. What is your motivation, and what are theirs? What do you want to achieve? What makes these guys that special versus finding someone (or at least looking) nearby?
Generally speaking, one of the foundations of poly life is belief in proactive communication. If you and your partners are not being proactive and honest, then yes, you're doing it wrong. The question then to ask is what you want and how do you make that happen?