(Miss Goodbody is away, today)
No matter how much I don't really care much about this matter, I do find myself getting a little miffed about the use of the word 'behaviour' to describe getting a boner, at the event. Unless the guy was 'helping himself' a little, then it doesn't count as wilful behaviour.
As Dikipedia (apols) puts it, "As an autonomic nervous system response, an erection may result from a variety of stimuli ... and is therefore not entirely under conscious control." Much the same as nipple arousal, or 'dampness', to put it demurely. It's just a bit more obvious for a bloke.
Sometimes it just happens

As I was explaining to the nice lady on the bus, this morning, when she was trying to reach over to ring the bell.