Sex Therapist and the Male Volunteer

A sex therapist student was doing research at the local college in town.
One of the male volunteers told him, "When I get it in part of the way, my vision blurs. And when I get it all the way in, I can't see a thing."
"Hmmm..." said the researcher "this is an interesting optical reaction to sex, Would you mind if I had a look at it?"
So the volunteer stuck out his tongue.