Ha ha! I love you both.

Well, I can offer two explanations:

1. His skin is supposed to be like stone so he's perpetually hard and always ready for penetration.
2. He drinks blood so even though he doesn't have his own, blood still runs through his veins.

I can't help about the semen... but nobody (including Stephenie Meyer) ever said vampires don't produce semen.

Go ask Stephenie Meyer.

They had sex when she was still human, and she gave birth to that mutt half-human half-vampire thing named Renesmee.

I was thinking about Jessica from True Blood.

Maybe that's where all the blood vampires suck is stored -- in their genitalia

Venom is what gets his dick hard. It also allows semen to still be in his body. Stephanie Meyer explained it all on her website.

Maybe he died with a boner so is permanently hard