Let me just say that I really feel for you - it must have been really hard to listen to your parents say that, especially since they're essentially judging your friends and your lifestyle without really understanding it.
I think we all have those moments where we sit down with family and then hear something really ignorant and unexpected. Sometimes, you can talk about it and present a more informed opinion, and they'll listen...and other times, you can't. They just made up their minds a long time ago, based on experiences they had, or gossip they heard. The expression "you can't teach an old dog new tricks" comes to mind...
At the moment, you are held hostage. If you push them too hard or try to make them see that they are "wrong" or "misinformed" they will likely only get offended and push back. They are your parents, and they believe they're doing what's best for you and looking out for your interests. Telling them they're wrong, even in a gentle way, could potentially mess up this last year of school for you.
So, I'd suggest you avoid the subject. Be yourself, as much as you can, and focus on what's good about your relationship with your parents. How nice it is to spend time with them, go on vacation (or whatever else you guys do over the summer), and find a way to tell your close friends that you won't be in contact with them much in the next few months, but not to worry, and that you'll explain in the fall. Don't use your email or cell phone if you can avoid it. Write them a letter - your parents are less likely to trace that.
I don't think you should try and put on a show for them, or lie about how straight you are...just avoid the conflict. Once you're back in school, and have finished your degree, maybe you can introduce them to one of your friends without telling them that your friend is gay. I find, if you meet someone in person, you're less likely to judge them. In time, your parents may come to see that you're perfectly fine and can protect yourself. Their thinking is just too abstract right now because as you said, they've never met a normal gay man, with a partner or with a respectable life.
I don't think they want to ruin your education. They're threatening to stop paying to scare you into what they believe is the "right" lifestyle. In time, if you take good care of yourself, study hard, find a job and respect yourself, they'll discover that their homosexual son is none of the things they feared a gay man to be
This will take time, though, so hang in there! Try to remember that they're just not seeing the whole picture. Forgive them that, and eventually they might grow to understand.
They tipped their hand by threatening to not let you go back. That shows that they care more about their own fobias than about your future. That should tell you a LOT about those people and about how quickly you need to become independent from them.
I've seen and heard about a lot of threats from a lot of parents, over all kinds of stupid stuff. Parents have threatened to "disown" their child if they date someone of another race. Parents saying being gay is "a phase" and that their child is "confused" and can't bring their partner home to meet them. The bottom line is, when a parent feels their child is in danger, they will do ANYTHING. They can even get a little crazy, because they're scared.
From what's been written, I don't feel like these people are willing to destroy their relationship with their son. They're just reacting in-the-moment. It sounds like this is the first time the topic has ever come up, and so their reaction is probably over-blown.
I agree that you should find a way to be independent as soon as possible, but I don't think you should feel betrayed and angry and hurt by your parents. Cut them some slack - they sound scared to me. They sound like they don't know how to help you be safe, and that's why they're trying the "hurt you to help you" threat of not paying for school.
This isn't the worst possible situation ever, so try and keep a level head about things. They're your parents, and you love them and they love you right? Stay calm, get through the summer, and be gentle in explaining that your friends really aren't using you, or tricking you. Maybe share some stories about the nice things you've done with your gay friends, but don't go into the fact that they're gay. Your parents aren't seeing them as people. They're seeing them as threats to their baby (you!)