While you may be getting an erection due to arousal, it may not be sexual arousal per se. It is common for males, particularly young males, to get erections at many different times. It's common to be sitting in class and get an erection for no reason whatsoever. It's common to wake up with an erection (meaning you have to urinate). I've heard that some men have even been known to get erections when they are scared.

Maybe you're just nervous about getting naked in front of someone else, and that in itself is an emotionally/senory overstimulation for you that leads to an arousal -- of fear, nervousness, anxiety, etc.

Erections aren't always sexual.