Sexpat is a (derogatory) term for sex tourists, foreigners that go to a country primarily to engage in prostitution. Sexpat is a devivation of expat or expatriate. Another term for sexpat is sex tourist.
The difference between a sex-tourist and a sexpat is the sexpat must LIVE in the country, a sex-tourist is a short stay visitor.
Sexpat (noun), an compound of sex and expat or expatriate.
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.