Translation:They will get married on Saturday
When the man is the subject:
ataoa = He will get married.
atamwoa = He will get married to her. / He will marry her.
When the woman is the subject:
ataolewa = She will get married.
ataolewa naye = She will get married to him. / She will marry him.
Both are the subject:
wataoana = They will get married (to each other).
-oana doesn't mean it's a same-sex marriage. It's necessary when both parties are the subject of the verb.
And while same-sex marriage is not a thing in Swahili-speaking countries, Swahili is not only spoken in Swahili speaking countries (eg. here in Germany), and also people don't only speak about things that happen in their country. To talk about same-sex marriage, I suppose a sure way not to go wrong when only one party is the subject of the verb would be to use the gender-neutral atafunga ndoa = s/he will get married (literally "s/he will close marriage").