First, the phrase is likely not Paul's; he appears to be quoting the/a
Corinthian position from their letter (a view which goes back at least as
far as Origen). For details see (among others) the commentary on 1
Corinthians by Gordon Fee (Eerdmans, 1987) 273-276.
For the lexical question, see Fee, p. 275, who offers nine occurrances
across six centuries, all of which, he says, refer to having sexual
intercourse (not simply "not marry").
Mike Holmes