Since when did trees have a sexuality

Whenever I have a maths lesson, there's always seems to be something random and unusual that goes on. Now, in the past 3 months, there have been complaining about windows being opened, our maths teacher constantly going in and out of our class during our lesson and people winding me up, only for me to wind them up and entertaining the rest of the class. Oh the joy! I am glad to say that this story i'm about to tell you does not involve me, thank goodness!

So there were 3 girls in my class, L, C and K. They are sitting at the row nearer to the window, and whilst my maths teacher is out of the room, we're doing the work when I hear:
C: L, is that tree falling down?
L: No, it's just bent?
C: I swear it looks like it's falling down!
L: Which tree are you talking about?
C: That one! (The rest of us look at the 3 girls, then the tree and then get back to work)
L: No, that's bent
K: It must be a gay tree (L and C are bursting out with laughter)
L: (Laughing) You seriously, make me laugh.
K: (Laughing a little bit) Sorry I had to get that out (Like she always does)

At that point, in pin-drop silence, all you could hear was L and C laughing.

[5 minutes later]
(Guess what, L and C are still laughing)
K: Gosh, it wasn't that funny. I won't speak next time. (I was so tempted to say, "thank's that would be great", but it was too cruel and instead, just stared at them)
(L and C stop laughing and try to get back to work but then laugh again. C burys head in her hands to contain herself)

Wow, I never thought thinking about gay trees made 2 girls laugh so much for about 5 minutes. K seems to make the girls laugh and when I have something random to say in the same level as K, they just look at me bewildered, and K starts to wind me up and I retalliate, a la "never mind the buzzcocks" style. It's sexism I tell you. I wonder if they'll laugh tomorrow when i shout out "gay trees"..nope, won't work.