Yes, because it's an accusation that the new lead singer AND the drummer are both gay AND a couple. Given that females will probably make up a sizeable percentage of their fan base, and some of those will have been drooling over Chase, finding out he's unattainable could be a cruel blow. I remember reading about the manager for the boy band Take That telling them that they weren't allowed to have girlfriends. They had to be seen to be available, even if they weren't. Instinct is in a different situation, as they are a rock band, not a pop band. They can be expected to be "wild" at times (which is why I'm curious as to exactly what Helen is going to jump on the guys about -- how much "wild" is acceptable and how much isn't?) but they are also supposed to fit the rock image. Gay couples aren't that common in rock bands and it may upset the image enough to cause them to lose fans or, worse, bookings.
Yes, the guys could deny it if the girls start to spread rumours (though Joe Lump could be called on to confirm them -- and that's a different situation to him outing the Chase himself so he might get away with it), but that will just come back and bite them in the future. The entire band and Helen KNOWS they are a couple and it won't be long before the roadies do, too. Trying to deny it would be a recipe for a disaster down the track.
I thought it was funny, too. He obviously hasn't seen enough of them. Having a kangaroo bound in front of your car as you're driving along a highway is scary....
It's tough to write a conversation when someone isn't in their normal state of mind. I'm happy to accept it as written. Dialogue isn't supposed to be real -- it is just supposed to sound real. Authors already leave out all those hesitations that real people use when they speak -- stretching things a bit when needed is just doing the same thing -- making it readable