That was a retcon John Byrne did in the 80s that was retconned away again more recently.
Stan Lee had them as childhood sweethearts, so the age gap was not that big before Byrne changed it. The more recent retcon by Matt Fraction had Reed meet Sue when he was at grad school and she was an undergraduate. Plus considering his genius he was most likely at grad school early. So the gap isn't that big or creepy anymore.