Equality is often not the friend of fairness. Fairness is both of you being free to explore. Equality is having to limit yourself until you are both at the same stage of freedom.
What would happen if you both fell in love and then he split up with his partner? Would you then have to split with yours?
I think your husband is absolutely a great example in how he accepted your need for this. There is a little bit of unrealistic expectations that your relationships should parallel his own however. Lives just don't work like that.
Sounds like you have a good start though. Keep communicating