When a straight man "goes gay" for the first time. The first time he has gay sex. He is said to tumble. Used especially in the case of very macho tough guys.
I'm into chicks and all, but I would totally tumble for Justin Beiber.
We all knew Fred had a bi streak to him, but it wasn't until he met Joey that he finally tumbled.
If you are looking for first timers check out that gay bar on the edge of town. It is known to have a lot of tumblers hanging around.