Speaking as a heterosexual white male, I can see that there is a difference between ‘real’ attraction and compulsory heterosexual attraction. I am also the straight version of the gay friend, so my experience may not be the same as yours. Lets try explaining with a few bullet points.

Are you interested in that person because you’re suppose to be so that you will fit in? You might be following compulsory heterosexuality.
Are you interested in that person because you’re religion only accepts heterosexual couples? You might be following compulsory heterosexuality.
Are you of the female gender and chasing after a friends brother when it’s really her that you can’t stop thinking about? You might be following compulsory heterosexuality.
Are you of the male gender and afraid of what the guys in the locker room will think if they find out that you like their butts? You might be following compulsory heterosexuality.
The question then become, how does one identify compulsory heterosexual behavior? I suggest the following three questions in an inner dialog.

Is my attraction intellectually or emotionally based?
Does the projected opinion of friends and/or family factor into the attraction?
What is it about this individual that I find attractive?
Though I know that it can be difficult at times, my last piece of advise is this. Always be yourself.