There is nothing to "believe". People have preferences and it isn't always
this or
that.
But I do have a question, do you consider for example, a guy who likes sleeping with boys but wouldn't want a relationship with one like he does with girls - a bisexual? Weird way of phrasing it but is one a bisexual if one prefers one gender for sex and one for relationships? See, it's all way too confusing and that is why I'm completely against labeling people.