I wouldn't be okay with it.
I have a boyfriend and the thought of us seeing other people would not make me happy.
We all need different people around us that satisfy our diverse needs (talking about this, talking about that, going to a yoga class, going to a football match etc.) but not in a romantical way. That what friends are for. And all the other people in your life.
I see the need for having to be with more people than your partner as being somehow unsatisfied. Wouldn't that mean that there is something wrong with your relationship?
Or with yourself? Maybe you're a nymphomaniac, maybe you can't deal with the issues in your relationship and take the easy way out, maybe you are not mature enough and not ready for this kind of a commitment....
People should really take a look at themselves. Whatever problems they have (in a relationship and out of them), they have it because something is not right. But looking deep inside yourself is what most people are afraid to do. They run from it. They run from the most intimate relationship they have because they are afraid of seeing themselves for what they are. They want to mantain status quo, they want to keep their familiar life, familiar habits etc.
I do not mind other people being happy in that kind of a relationships but I would have a problem with it.
Being in a relationship for me is to be with someone who makes you happy and makes you realize who you are. Staying in a relationship that is really intimate can change you for the best if you are strong enough to stay in it even when things go bad.