What do you mean "how do they have sex".. it's not like being bi gives us completely different "parts" down there.. Being bi simply means that. they are attracted to both guys and girls, no matter how feminine or masculine either one is .
How do they love? Well, it means they can fall in love with either a boy, OR a girl, rather than just strictly a boy, or a girl, depending on their sex.
How do they move between different genders? They just find both attractive, like I said!