In modern Chinese,
勃起 is overwhelmingly the medical / clinical term for an erection — neutral and non-vulgar, as used by doctors and pharmacists (e.g.
勃起功能障碍 ED,
勃起药物 ED medication). It does have a literary sense "to suddenly rise / spring up" (industries, movements), but that usage is rare and bookish; readers will assume the medical meaning unless heavy classical context says otherwise.