Often times, what makes insertion difficult is the man’s inability to relax. Let’s face it, if he keeps clenching, there’s no way you’re ever going to get the strapon in without a great deal of pain.
However, if you make him place his toes together and his heels apart, then bend him over a table or counter, then you will have no problem inserting a properly sized strapon dildo. Why? Because in this position it is not possible to clench the anus muscles. That’s why doctors use that position for rectal exams.