How can a vampire have sex if they don't have an erection?

I assume that a vampire can't have erections because all of their organs are dead, therefore their heart can't pump blood through their bodies which is what is needed to become erect. Are vampire just naturally "excited" and don't need to spring up? Do they just have sex without being erect? I just don't know. I am trying to write a fantasy story where a girl (actually a witch) falls in love with a vampire and they eventually get married. There's definitely way more to the plot than that, but because it's not on topic, I'm not going to go into it. Anyway, my characters obviously have sex, (and I'm not going to write out their love making, of course) but I just really need to know how it works. I don't want to use the explanation the Stephenie Meyer used, saying the vampire's venom replaced the blood and semen. (my vampires can't procreate) If any of this is confusing, you can just ask and I'll try to explain myself better. Any response would be great. Thank you!

They just have sex the same way human dose. When a pure blood vampire have sex with a human. He or she could turned into a pure blood vampire. I got this answer from Vampire knight. I know it sound silly, but worth a try!