Tldr, not worth the energy.

Think about how you’ll feel in 5 years looking back, and you’ll appreciate waiting for the right person.

I’d recommend trying to have a mature conversation with Ben, and if it goes well, great! If not, then it’s keeping Ben at an arms length and not letting them get closer. And advising the same for Beth.

As long as you try to stay cool headed, you shouldn’t do something you’ll regret later on in life. As long as Ben can have a mature conversation about relationships and “ownership” of relationships, and how language can hurt people’s feelings (which is kindergarten shit, Ben…) you should be able to maintain a decent friendship. If you’re cool about it, at least you can say if things go south it’s Ben who is/was the problem.