The design of the study has more to do with this than anything else. Still, there's been several decades (at least) of independent study on the subject, and all of them i've seen conclude that more men cheat than women, even if more recent studies show the frequency of women cheating is increasing. Regardless of your math, the fact that every study leads to the same conclusion is what I would call strong evidence that this is more likely than not to be true.

The simplest answer to your question ("who are all those extra men partnering with?") is incredibly simple: there's a smaller group of women who sleep with a greater number of men than vice-versa. And this makes total sense if you consider that women have more choice of partners, and men have less, and that there's a larger pool of men looking for purely physical/sexual relationships. Basically the availability of men is greater, the availability of women is smaller, and so the reported number of women cheating is smaller, and the reported number of men cheating is larger.