Do you mean if s.o. Is prosecuted in a state where the age of consent is say 21, the court should ascertain if the accused had sex with anybody under that age,in other countries or states? In one southern state a man can legally marry a 14 yr old. (Need to search that)
what if he married a 15 year old girl in France? Legal there. What if he/she had sex with a 14 yr old boy in Spain, or married a 14 yr old girl there? Both legal in Spain. ( Age of consent there for all types used to be 13 - until Germany pressed for aligning age of consent throughout the EU )
If the person involved isn't resident in the state concerned should he be extradited for breaking a US law? This is a can of worms.