An erection usually follows arousal. You can get aroused without an erection but the context of "degree" is important. Well, it is if you're not a man that doesn't have problems gaining erections.
So, for the man that has no erectile dysfunction problems, past a certain degree of arousal
not gaining an erection is impossible.
(I'm sure here enters the male feminist with some BS about how "mature" men of the 21st Century can hit the peaks of arousal and orgasm just by cuddling with women, and all the while never gaining an erection because they have "control" over themselves
)
Some things about the human body are involuntary, like your heart beating, or small cuts healing. The male erection in relation to arousal falls somewhere in the middle of what a man can and can not control. It depends on the degree of arousal.
I still get non-arousal erections that are totally involuntary. I've gotten them sitting on public buses, in classrooms, just about everywhere. Have no idea why they come when they do but they just do. And that's
without sexual arousal.
Feminist men and women might complain, "Why all males control their abilities to get erection 100% of the time?" and inquire with disdain. My question is why don't females control their tw__s from bleeding 100% of the time? Eh? Never mind.