It is clear cut. Either you are straight, or homosexual. Being bisexual is a form of homosexuality. As you stated, homosexual is abnormal. Since there are only two options, and homosexuality is the ABNORMAL form, heterosexuality, is of course, the NORMAL form. It is necessary for there to be a normal sexuality, in order for there to be an abnormal one. Without a normal sexuality, it would be impossible to realize an abnormal sexuality. By admitting homosexuality is abnormal, you admit that heterosexuality is normal.
For one to conduct in abnormal activity, makes them an abnormal person.
Someone using their left hand is not the equivalent of pronouncing a homosexual abnormal because left handed individuals preform the same tasks as right handed individuals. Right handed individuals use their left hand in addition to their right hand. For example, I am typing with both hands right now, but I am right handed. A Normal (which you admitted to) heterosexual person doesn't use "both hands."
The activities of a left handed individual are the same as right handed individual's, except they use their left hand more often. They however, also use their right hand, just like right handed individuals also use their left hand. They aren't even close to being equivalent.