That still lets Alice find out. I don't see a clear way of mapping the concept of a zero knowledge proof to the crush problem. But the best I can come up with

That still lets Alice find out. I don't see a clear way of mapping the concept of a zero knowledge proof to the crush problem. But the best I can come up with is: 1) No one learns that Alice (claims to) have a crush on Bob 2) Unless Bob also claims to have a crush on Alice, in which case. 3) Only Bob learns that Alice has a crush on him. 4) If Bob learns that Alice has a crush on him, Alice learns that Bob has a crush on her. 5) All the above guarantees are symmetric if you swap Bob and Alice.