I was bored so I did some math. Assuming a probability of 5% of being homosexual, in a group of 40 people, there's an 87.1% chance at least one person is homosexual and a 60% chance at least two are. I wouldn't call 60% likely, if this were before Jun was outed to his classmates.
But since we're given that there's already one person who is homosexual (ie Jun), in the remaining 39 people or so, there's an 86.5% chance there's at least one more. However, there's only a 27.8% chance that there is exactly one homosexual out of the remaining 39 or so. Onoie is a bit confident don't ya think? He's talking as if there's exactly one other person. /s. Or maybe just a translation thing.