Recently it has become possible for men and women to marry someone of the same sex and easier to raise children together, though many barriers remain, depending on the location. But the traditional view of marriage and child-rearing involves a man and a woman, and many people do not want to give up that option.
If you have the capacity to enjoy a sexual relationship with both women and men, you might prefer to stay on the more straight-forward road to your domestic goals. If you declare yourself bisexual, you may be viewed by potential future partners as deviant, unreliable in your commitment, and possibly gay. Potential spouses might not consider your flexible sexuality a strong basis for a life-long relationship. Even you might worry about how you would explain your bisexuality to your children (though hopefully such concerns will seem silly a generation from now).
On the other hand, you may not want to spend your life building a family with someone who is hostile to your experiences and interests, and from whom you have to keep secrets.
Flexuality 