Relationships SHOULD enhance your life. So yes, your relationship with him enhances their marriage. Honestly it should. If you enhance his life, then he's happier, that is going to effect those around him, including his marriage. Guess what? It works the other way too. Think about it, when you spend time with him is he happy or is he in a bad mood because of things going on in his marriage? If someone has a bad day, it affects them.
A mistake we made in the beginning was trying to put each relationship in it's own little box. As if one didn't affect the other. Of course they do! If BF and I hare having problems, just frustrated or dealing with something, hubby is going to notice, he's going to see I'm upset or unhappy or frustrated. And vice versa. You can't have each relationship to completely independent that there is no cross over.
One of the things I LOVE about the openness of the relationships is that I CAN tell hubby something good BF and I shared and vice versa. There is no upset like, "Well I don't want to watch that movie now because you two did and had fun so my fun is lessened watching it." Instead, enjoy how the relationships enhance each other. It's not a one way street. You aren't just some 'tool' to be used to make his marriage better, any more than his wife is just someone he stays with because it makes YOUR relationship better. He loves you both, he wants to be with you both. Probably because of how you both enhance his life and I'm sure he hopes he enhances yours. The effort you are afraid of putting in and going to his marriage and not to you is effort you put into YOUR relationship. If you aren't getting enough out of YOUR relationship for the effort you feel you are putting in, say something. But don't assume it's like some sort of relationship bank where you are making deposits and the wife gets the withdrawals!
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year