The main consequence of having visited Iran before going to the U.S. is that you are no longer eligible to travel on the Visa Waiver Program (VWP).
If you have travelled to or been present in Iran, Iraq, Libya, Somalia, Sudan, Syria, or Yemen on or after March 1, 2011, you will need to follow the regular process and apply for a visa at a U.S. Embassy or Consulate.
While not being able to travel on the VWP is a hassle, it is likely the only consequence. That you would not be able to visit the U.S. after Iran is improbable (given, of course, that there are no aggravating circumstances).
See also How long does the US Visa Waiver ban for people who travelled to Iran last? and I have a valid visa for the USA. I have visited Iran since it was issued. Can I still enter the US?
Source: https://travel.state.gov/content/visas/en/visit/visa-waiver-program.html
See also this answer by Zach Lipton:
Therefore, there is nothing that would prohibit you from coming to the US. It is, of course, possible that you could receive extra questioning at immigration, and if you're seen as a security risk, you won't be admitted. That's always true whether or not you've visited Iran though. A visa is never a guarantee of admission.
From I have a valid visa for the USA. I have visited Iran since it was issued. Can I still enter the US?
See also How long does the US Visa Waiver ban for people who travelled to Iran last?.