Usually in English we talk about a language being someone's "mother tongue"... unless you mean it was the language of your father, in which case you could easily say (and everyone will understand you!) "Polish is my father's mother tongue".
But maybe you were just being jokey, or creative :D
You refer to the fact that most feminine nouns have the same Genitive singular as Nominative/Accusative plural, right? Yeah, that's true for most of them. But this is really not the case here.
Firstly, "mężczyzna" is masculine, because, well, logic. It means "a man", it would be absurd if the word wasn't masculine ;) There are several nouns describing masculine people that end with -a in their basic form. Including "tata" (dad).
The Genitive form "mężczyzny" looks like something from a declension of a feminine noun. Compare to "starszyzna" (a collective noun more or less equivalent to "the elders"). "Nie lubię starszyzny". Looks almost the same, although "starszyzna" is feminine.
As for plural forms, they are different. We have two plurals, 'masculine personal plural' and 'not masculine-personal plural'. This word, when in plural, obviously belongs to the first category. The Nominative form is "mężczyźni" and both Genitive and Accusative forms are "mężczyzn".
Pls look into https://claritaslux.com/polish-cases/ If you say for example that you like or see someone, you need to use an accusative case. But if you want to use an opposite (denying) sentence you should use a genitive case. It is hard to explan the reason behind to non Polish native speakers...
"lubić" takes Accusative: "Lubię tego mężczyznę".
If a verb that takes Accusative gets negated, it takes Genitive instead: "Nie lubię tego mężczyzny".
-y can be a plural ending, but it's also a Genitive singular feminine ending. Sure, "mężczyzna" isn't a feminine word... but it looks like one and undergoes declension like them.