First off, "в жены' is accusative plural (not nominative).
The expression 'выбирать в' always needs a plural form, even if you mean only one person. "выбирать в жены / в мужья" (but: "выбирать мужа / жену); выбирать в депутаты, выбирать в друзья. Выбрать кого-то себе в учителя -- to choose a teacher.
I think it implies not the plural form by itself, but the general idea 'of being someone', 'belonging to a particular group'. Like: there are women, some of them are wives, you're becoming one of them = I make you one of them.