What has often happened since the 1970s, the time feminist theorists made convincing cases for the masculine gender not being universal, is that gendered terms have been replaced by gender neutral terms, thus solving the problem. We seldom, for example, speak of policemen any more as the preferred term is police officers. Likewise, firemen have become firefighters and workmen workers, while housewives are now homemakers. This solves not only the original primary problem of binary gender identity but also allows for gender fluidity.
Manhole cover is a more difficult proposition as it describes a thing, not a person. The best way to deal with a term like this is think about the function of what is being described. Manholes are called manholes because they are holes constructed to allow a person to get under the street easily. They provide access. They were originally called manholes because it was assumed only men would be hired to use them. Now, they could simply be called access holes or covered access points to describe their function.
It can take a little while for people to get used to new terms, but usually a focus on function helps. It is also encouraging to keep in mind that English, because it has been open to incorporating words from many languages for centuries, almost always has a number of choices to convey the same meaning.