Why is so fragile of bisexual relationship?

Bisexual means someone love both women and men.
According to some bisexual experience of bisexual people, the bisexual relationship is very fragile. But why?

At first, we should know the essential difference between bisexual and straight is that bisexual may fall in love the same sex. 
Bisexual usually is compared with straight and gay by many people. They think bisexual is a synonym for sexploitation. What is sexual intercourse? Sexual intercourse is not just for have a baby, more important is for happy!
There is scientific evidence that men's sexual desire is higher than that of women and in the relationship between men and women, usually, men are more active. This is the internal factor that the bisexual relationship is very fragile.
And on the other hand, bisexual relationships are not protected by law, except for a man and a woman married to become bisexual couples. Some experienced divorced friends say that they tend to break up with her boyfriend over the phone talking to. The lack of marriage contract is one of the important reasons for the vulnerability of the bisexual relationship.
So, when a bisexual love you, please cherish. They really love you!