It might be bothersome for the both parts of the relation if one "separates" from the partner to watch other persons. I mean, you have a sexual life and you see your partner "leave" you and masturbate with other persons. Wouldn't that lead one to think you are not satisfactory or pleasing enough? That's for sure.
It could be sacrificing, the well-being of the other part, for the sake of the other. I personally consider it like cheating. It's about respect for the other, that can hurt one's feeling unnecessarily.
However, the best number one thing for a couple is communication and that might be something interesting to talk about, as long as it's not troublesome.
Both me and my partner have a healthy relationship and a healthy sexual life and refuse to do something like that, it's just like I said above, and we love each other so much it's enough not to have interest in anyone else. Also, it's a sign of respect among us. And, we just can't do it. Basicly, it's just like cheating on her.
If I want to masturbate, I ask her to do it, if she wants to do it, I would do it for her. Simply enough.