There is no excuse for it, but there are factors. McQueary knew Sandusky. When McQueary was the starting QB for Penn State (and McQueary still holds a lot of Penn State records), McQueary was the defensive coordinator, and Penn State was Linebacker U. McQueary knew that Sandusky had been a coach at Penn State since before McQueary was born. He likely knew that Sandusky had played for Joe Paterno, and he certainly knew that Sandusky had been friends with Joe Paterno since before he was born.
McQueary knew that Sandusky had coached at Penn State for 30 years.
McQueary knew that Sandusky was so famous that he had been named the NCAA Assistant Coach of the Year in 1986 and 1999 (yep, the year after the investigation by the University Police into incidents of showering with little boys, and the year Sandusky retired).
We're removed from it now, because Sandusky retired in 1999 . . . but Sandusky was a legend. Not just at Penn State. Sandusky is considered one of the greatest defensive coaches in NCAA history.
So THAT is who McQueary walked in on. He didn't walk in on some 55 year-old man that he didn't know. McQueary probably knew about the 1998 and 2000 problems that Sandusky had - but if he did, he knew that Jerry was being protected with his emeritus status.
But college football is a religion of sorts for a lot of people, let's not kid ourselves. And for more than 25 years at Penn State, Jerry Sandusky was a demi-god, second only to Joe Paterno.
So Mike McQueary had no excuse . . . but he had a lot of reasons.
His future at PSU.
Here is a great article by someone who grew up in Happy Valley that discusses just how incestuous the situation was.