The belief alone does not make someone phobic, but actively going out of your way to try to convince gay and lesbian people to consider having sex with the opposite sex IS homophobic.
Some people are like that. They see what they believe to be a social injustice or people otherwise needlessly missing out, and they feel like they have to flap their gums about it.
I will agree that there are ways of doing things though. Having a general discussion about sex outside of marriage, or the phenomena of people who are homosexual or bisexual in orientation but who exclude the same sex from their dating pools, or those who exclude trans, is a different kind of thing than pestering individuals.
If a specific individual in the 70s tells you they do not want to have sex outside of marriage then it would be wrong to harass them about it. If a specific person you suspect is a repressed homosexual or bisexuals tells you they only want the opposite sex and does not want to discuss it, then leave them alone.
But the general phenomenon is bigger than specific individuals and their specific circumstances.
There I cannot agree that it is "homophobic" or "heterophobic" or anything else just to believe what you believe and to engage in general discussions about it, release Youtube video pieces on it, etc.
In fact still not sure I would call it as homophobic to harass a specific homosexual you suspect is letting prejudice or conditioning interfere in their exclusions, that it is heterophobic to do the same to a person you suspect to be a repressed homo or bisexual.
But it would still be wrong to pester individuals, of course.