Yup. Both boxes can be checked.
From the official blog post at
https://ea.com/en-gb/games/the-sims/the-sims-4/news/sexual-orientation-in-the-sims-4
"You can check any combination of attraction boxes (including all or none) to reflect your sim's romantic orientation. Further down the line, we'll be able to expand this to include additional gender identities that we don't have at this time."
The question that is being asked for that section of the panel is "who is your sim attracted to?"
So, if you click both boxes, they will be attracted to both genders. Click neither box and they will be attracted to neither.
That same blog also says this:
"Sims, prior to Sexual Orientation essentially use the above setting. They have no inherent attraction to a specific gender, they can have their attraction shift through gameplay, and they will WooHoo with any gender. This is still the default state for a new Sim."
So unless you specifically change the settings for a sim, nothing changes. They will still behave exactly as they always have.
