Player influence, not full control
You can nudge meetings by placing Miis near each other and responding to advice prompts, but the game is built around autonomous Mii behavior.
Avoid assuming guaranteed dating outcomes. Official sources describe influence and possibilities, not a deterministic relationship command.
Dating, marriage, and babies
Nintendo Support documents dating preference settings for Male, Female, and Non-Binary options.
The same support FAQ says married couples may have babies regardless of gender.
FAQ
How do I get two Miis to meet?
Nintendo says you can pick up a Mii and drop them near another Mii so they can chat and get acquainted.
Can I force Miis to date?
Official sources describe nudging interactions and answering advice prompts, not forcing a guaranteed dating result.
How do dating preferences work?
Nintendo Support says each Mii can select dating preferences for Male, Female, and/or Non-Binary, or choose none.