Is Section 2423(b) of the Civil Code of Civil Procedure unconstitutional because it prohibits travel in interstate commerce for the purpose of engaging in any sexual act with a minor?

Section 2423(b) forbids traveling "in interstate commerce . . . for the purpose of engaging in any sexual act (as defined in section 2246) with a person under 18 years of age." Han claims that this statute, as applied to him, is unconstitutional because the statute "requires merely the crossing of state lines with illegal thoughts to effectuate federal jurisdiction." He argues that United States v. Lopez, 514 U.S. 549, 559 (1995), requires an inquiry into whether the regulated activity "substantially affects" interstate commerce. The government contends that this requirement is not applicable to the case at bar.