The method simulate.sisnetwork simulates graphs with the same marginals as the passed network or as the rows and columns specified in a sisnetwork object through sequential importance sampling. That is, the degrees of the nodes are fixed and specified.
networksis-package
(Package: networksis) :
Simulate Bipartite Graphs with Fixed Marginals Through Sequential Importance Sampling
The networksis package is a collection of functions to simulate bipartite graphs with fixed marginals. For a list of functions, type: help(package="networksis")
providing 
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