Sometimes this can be embarrassing, especially when you first meet someone. Even if you aren't talking about anything sexual sometimes the brain is just wired wrong. If you're sitting down obviously you can hide it, but in certain circumstances it just pops up uninvited, like she mentions boring talk about work but reminds you of that filthy girl in XXX you watched the night before.
  That comedy Something About Mary one person suggests yanking one out beforehand to get your mind off the subject. Other people suggest avoiding all aphrodisiacs and watching porn less than one hour per week. Apart from chemical castration, and self-flagellation what do you do to try to prevent Wily from getting too excited?