the illicit sexual conduct must be: an "efficient and compelling purpw: I15 Fid 1488. 1495 (106 Cir. 19517): • "motivating purpose," United States Cagithli. 49 F.3d 1079, 1083 (56 Cif( 1995), or "at least one of the defendant's motivations for taking trip in the first place," United Statist I EU& 935 F.