Anytime people see a man and a woman together, they assume the couple is heterosexual.
Likewise, a same sex couple is automatically thought to be gay. But the fact is that any or all of those people could be bisexual. Bisexuals largely go unnoticed for numerous reasons. Among them, America likes its absolutes. When asked of your race, gender, religion, political party, or sexual preference, you rarely have the opportunity to check multiple boxes.