Seriously, who wants to be tied down?

Since she was 'born' Succubus (and since most males seem to like feminine wiles), she prefers being female over male. If you're Bisexual, I would say... go for two. The game doesn't go in depth how powerful she is, but considering how long she's been around, she should be able to project being two people if needed. However, this will limit her being able to fool around with more people.