It's a very common fear and it's ok for you to be uncomfortable about coming out to them. But it's really encouraging to know that they are true LGBTQ rights supporters, it means that they value LGBTQ people and accept them for who they are. So why shouldn't they do the same for you? Just try to be as open-hearted to them as you can, explain how you feel and how much it would mean for you to have them by your side. If they accept and value LGBTQ people, there's no reason why shouldn't support you. And in any case, you'll be there to answer any question they might have, solve any doubt... Dialogue and openness are the key.