I just asked my husband, and we had the same response. It depends upon the people, of course, but neither of us would have a problem with the other going out alone, if the motive is to have a good time, speaking to the reason they’re going out. If the motivation is to find another person, then of course we would have problems with it.
We have a stable, solid, committed relationship, though, coupled with a healthy realization that we ultimately don’t have control over all situations; we’d be complete idiots to think that it could never happen. There are risks to everything that we do, but to focus only on the risks is dangerous and unhealthy for a relationship. Ignoring the risks completely is equally dangerous, but we work not only on our relationship, but ourselves as well, realizing that life and self development are ongoing processes. That’s probably what has helped us the most and allows us to fully enjoy life and each other.