Who you're interested in are checkboxes. You can click either, both, or neither. They openly said that asexual sims are possible, so it makes sense that clicking both boxes is as easy as clicking neither.
Whether you're open to explore is radio buttons, so you can only click one. So a sim might tend one way or the other, but not exclusively so.
Sims will default to bi. And many sims who are portrayed as straight or gay will continue to be bi in practice; Brant and Brent Hecking are canonically gay, Don Lothario is canonically straight, but the game won't stop you from romancing any of them. What this counts as is up to you.
