If soliciting Internet sex do the state laws of the asker or the askee apply?

This thread re the IM’ing Congressman made me wonder how they parse that out.

Lets say the “asker” is in a state where the age of consent is 16 and the askee is in one where it’s 18. Or reverse that. Assuming the two participants are in different states with different AOCs how do the authorities decide which state law applies, or is it some federal thing?

It seems that it is a federal law. IANAL, but wouldn’t federal laws override most state laws? From this site and a lot more out there: