I would except the erection picture as erotica if the mood is matter of fact rather than coy or lavascious but I don't think any formal guidlines or TOS would except the erection as not pornographic. I have painted the artis's model with erection but no lewdness... I know it can be done... but "The Male Nude in Contemporary Photography" by Melody D. Davis discusses in a very interesting way how the male hedimony tries to control representations of the erection and why!

As an art student I avoided painting women because at the time I was very sensitive to issues of exploitation but my images of men in classical poses was considered subversive and even enraged some people... when I was first exhibited comercially in New York city there was an attempt to pidgen hole me as a Gay Artist although my nudes were not sexual. Some gay men seemed somehow "disapointed" when they realized I was not gay... but what was really so powerful was that reaction by some people to my nudes back in art school: anger! I had chosen the male also because I wanted to not fall in the trap of making the figure beautiful or being erotically attracted to the model, but when I gave up exhibiting and became a teacher I started painting women... pretty women.