Can a 21-year-old get in trouble for helping his pregnant 17-year-old girlfriend cross state lines to marry?

Well, it doesn't appear to be a violation of the Mann Act of 1910. http://www.usdoj.gov/usao/eousa/foia_reading_room/usam/title9/crm02027.htm

...transporting any individual, ...for the purpose of prostitution or sexual activity ...transportation of minors under the age of 18 years ... provides for an enhanced penalty.

See also emancipation http://www.steveshorr.com/child.support.military.service.htm#Emancipated%20minor;%20description

heaps of people have done it so i dont think it would be a problem good luck