It's nothing to do with what you get up to in bed Alan, it supposed to symbolize your long term committment to the barangay, or at least thats what they told me. But as they charge you a couple of hundred pesos for this and they send a guy along with you to do the planting who your expected to tip as well, I'm pretty sure it's just something that someone has dreamed up to take some more money of the white man.
Iain.