I will be honest..

I can always get hard when wanking. I never lose erection mid way
I don't lose erection during sex. Its just the build up to sex that I've lost it...

I can always get hard to porn, and yes, even if I didn't want to..

So yeah, its just when theres a girl, and I'm nervous or worrying or something..

I hope I've entertained on your day off