Plural marriage means polygamy or polygyny. Basically, it can mean, for example, having more than one wife.
The word marriage is not a plural form; the plural form for the noun is marriages. Or, perhaps you mean monogamy, the opposite of polygamy.
Marriages.
a man with 10 wifes
Yes, the noun 'marriage' is a countable noun. The plural noun is marriages.
Means is already plural. The singular is mean.
Plural Marriage.
If you mean handkerchief, the plural is handkerchiefs
If you mean vulva, the plural is vulvae.
If you mean "Army" then the plural is "Armies".
If you mean the plural of mouse it is mice.
Do you mean "plural of party?" If so, the plural of party is parties.
There is no such word. If you mean thankful, there is no plural.