In most people's minds, the heterosexual people would only fall in love with heterosexual people.

I think this view is incorrect or incomplete. Generally speaking, most straight will only get married with straight. But we know that when it's faced with true love, nothing is impossible.
Now let's discuss the problem from the other side. Why so many bisexuals will choose straight to married?. I think it's may affected by three points.
1. The most important reason is social pressure.Come out as bisexual is difficult for anyone. Even your family supports you. But when you're in public, a lot of strangers may give you a curious look. Even 99% of them have no malice, they're just out of curiosity. This unconscious behavior is the best description of their minds. They showed curiosity that made them think it was abnormal. At least they've separated themselves from you. It has nothing to do with heterosexuality and heterosexuality. But for gay and bisexual, where you go, there's a sense of being watched isn't wonderful.

2. The establishment of a bisexual family is very difficult.
As mentioned earlier, if your family supports you, you have a bisexual partner, and you don't mind other people's eyes. Now, you beat 50% bisexuals in the world. The next step you should do is to find a partner. According to your preferences, you can try to find a girl / boy / two. After you come out of the closet. There are lots of parties you can take part in. (for example, Meetup website, online sexual dating site, Facebook community). But the key is that you must pick up your love from those who bisexual. Make sure they love you too. You can analyze the possibilities of success. Maybe bisexual party website can help you a lot.

3. Some straight would like to experience bisexual life.
Along with the social opening, more and more people accept homosexuality. At the same time, they were curious about homosexuality. Many young heterosexual and bisexual to date. Some people find love when they're dating. Therefore, more and more people are willing to join the bisexual straight "mysterious world.
Sometimes in order to get the things we have to give up some things.

When a bisexual and straight encounter love, the time is right to get married.