A sexpat is one who participates in tourism with the express intention of having sex.
There may be a variety of reasons for this, such as:
less restrictive laws in foreign countries;
anonymity or privacy;
a preference for people of other ethnic groups;
financial reasons i.e. if one travels from a wealthy country to a poorer one.
However, by far and away the single greatest reason for the existence of the sexpat phenomenon is that sexpats find securing carnal relations with someone in their own country difficult to impossible to achieve.
Andy: It's because of all the nasty old ***man sexpats over here.
Shane: I agree. Shall we go to Patong and ogle some bar girls?