Im sorry your experience has been difficult. I hate the term secondary. I beleive you are in a V relationship with the guy as the hinge. I am in the same relationship and I am also the OSO. I had some of your same concerns yet not to your extent. Rules and conditions have to be discussed ahead of time and even then they may get misunderstood along the way. Positive,open, honest communication is what helps to sustain IMO. Maybe you can talk to him tell him how you feel, let him know what your feelings are.
I didn't get to negotiate ANYTHING about the relationship
Would you accept this in any other relationship? (ie family trip,business)
she seems to view me as a sort of free service provider that takes care of her husband's sexual needs for him so that she doesn't have to.
Until you said those words I didn't realize that this is something I provide also but I also get other benefits of an actual relationship that I cherish and enjoy. You have to remember this is your relationship also, and If you are not getting what you want out of it, then express that and possibly renogotiate the terms of the relationship.
What do I know Im just a beginner, some really awesome people will be along shortly.
Best of luck to you!