id say they are maybe either bored of the relationship, as their mind is on other things
or, that they havent fully "found themselves" yet, and arnt sure if they want to be in a straight relationship yet. Ie they are thinking about men, women and all sorts of different stuff.
either way, and i know this will sound harsh...im not convinced that relationships can last in these situations. As someday the gf/bf will want to experiment and obviously you cant offer both genders for him/her to satisfy themselves with, theyd need an "affair" so to speak, with a boyfreind or girlfreind. So even the most open minded people wouldent be able to cope with that IMO