There is this idea that, by "putting out" for multiple people, consent means less. As though spreading it thins it.
Another way of looking at it is that sticking to one person can become an obligation, a chore. Enthusiastic consent can lose its enthusiasm in stagnancy. Enjoying different partners allows one to value each more.
Neither point of view is generally right or wrong, just different for different people. Giving enthusiastic consent to one person can strengthen it for some people, sharing it with different partners empowers it for others.
Enthusiastic consent is not a dwindling resource. Whether given to one or to many, frequent giving makes it more joyous. The enthusiasm makes one an active partner, fully aware of what's being shared.