What does "se hunde la cama" mean?

Soy un viejo, ya he pasado los 44, pero cuando se hunde la cama, acabo en el suelo.

What does this mean? I understand all the words, but it supposed to be a joke and I don't understand it.

I'm an old fart, already over 44, but when the bed floods, I jerk off/come at the floor?