cit sexual conduct must be: an "efficient end compelliog purpos United States' Meacham 115 F.3d 1488, 1495 (10°' Cir. 1991); a "motivating purpose," Unitt. Campbell 49 F.3d 1079, 1083 (5° 1995), or "at least one of the defendant's motivations for taking the trip in the first place," United States I.