This is not a betrayal of anyone, and please please do it!
First of all you aren't betraying anyone's trust because the BFs parents aren't meeting with you to ask you to protect their son from having a bf, their meeting with you to determine if your son is trustworthy with their son. There was no promise or expectation that you would inform them of their sons sexuality. Your role in this meetup is identical regardless of rather they are friends or more and then that, and so are your expectations. Seriously there is absolutely no lie here.
In fact your refusal could almost be seen as a double standard here (note, I'm not saying it is, only how it could be seen, relevant below). Would you have such a problem if say your son wanted to meet with a girl who he wanted to date but the parents thought the relationship was just friendship? Lets be frank kids have crushes all the time, parents often don't know who kids are attracted to and most kids prefer that, would you feel obligated to tell parents any time you knew a kid had a crush on another kid? Is the problem really that the two like each other as more then friends, or that you feel bad in supporting a kid in hiding his sexuality from parents he doesn't yet trust to be open with? Because if it's the latter, the parents have no right to that knowledge to begin with unless and until their son is comfortable sharing it!
To be frank it can be hard to be gay, it can be extra hard when you are in the closet and afraid to tell anyone, and triple so if you can't trust even your own parents to support you. For this boy it likely feels like the world is against him and no one supports his sexuality. Perhaps your personal concern is just that you feel it's wrong to paint a relationship as a friendship and it has nothing to do with their sexuality, but that isn't necessarily how he will see it. To that boy you will be yet another adult who refuses to support him, his sexuality, and his desire for a normal relationship. Just one more piece of evidence that it's not safe to come out because people can't be trusted to support who he is.
Whatever your personal motivations for being uneasy, it will look bad to the boys if you don't support them. It could potentially even harm your sons belief that you fully support his being gay if you fail to support his first real gay relationship.
Put simply the potential harm to the boys of having you not support them in this, whatever your motives, is far far higher then any theoretical harm to the parents not knowing that their son has a crush on someone. The very real need of both kids to have you support them in their first relationship is a much much higher importance. I'd plead with you to meet with the parents in this situation.