Heterosexism is the assumption that all people are heterosexual and that heterosexuality is right, correct, and normal. Both can impact each one of us at different times and in different ways. We aren't the cause of homophobia and heterosexism, we are their targets.
Heterosexism is just one form of oppression; oppression is the unjust exercise of authority and power by one group over another. It includes imposing one group's belief system, values and life ways over another group. It's the way our mothers and fathers talk to us about providing them with grandchildren, it's the way other people ask us about our girlfriends, it's the way people just assume and expect us to be heterosexual. It's all those societal norms and pressures to be straight, and masculine. It causes us to believe, and act, as if someone else's belief systems, values, and way of life, is what we should strive for.
"Homophobia can create fear and anger that can last our whole life. Being gay is not wrong; in fact it's great. It's your right to be whom, and what you are. What is wrong is when you feel you have to be something that you are not" - (Gender and Sexual Identity: The Journey Begins, 2000, Canadian Strategy on HIV / AIDS, Health Canada, p. 2)
Homophobia is the irrational fear of homosexuals, homosexuality, or any behavior, belief, or attitude of self or others, which doesn’t conform to rigid sex-role stereotypes. It is the fear that enforces sexism and heterosexism. The extreme behavior of homophobia is violence.
There are some really great websites that are useful in working towards ending and out living homophobia. It's good to have allies and resources that can help guide us through the difficult times. Being gay is becoming more socially acceptable than it was even 10 years ago. We have a history and even a history month. In Montreal there are even archives dedicated to gay folks and our history, and another one in Toronto.