How can they have sex?

well, even if they are bloodless, that does not mean that they do not have the ambition to be physically active. They still want to be like humans, so of course they are going to do things that humans do. It's hard to explain, so sorry if this answer confuses anyone lol. But this idea isn't a blooper.

well in the context of the fantasía story this is how it goes:

Throughout the vampire's body are many versions of venom-based fluids that retain a marked resemblance to the fluid that was replaced, and function in much the same way and toward the same purpose. Though there is no venom replacement that works precisely like blood, many of the functions of blood are carried on in some form. Also, the nervous system runs in a slightly different but heightened way. Some involuntary reactions, like breathing, continue (in that specific example because vampiros use the scents in the air much más than we do, rather than out of a need for oxygen). Other involuntary reactions, like blinking, don't exist because there is no purpose for them. The normal reactions of arousal are still present in vampires, made possible por venom-related fluids that cause tissues to react similarly as they do to an influx of blood. Like with vampire skin—which looks similar to human skin and has the same basic function—fluids closely related to seminal fluids still exist in male vampires, which carry genetic information and are capable of bonding with a human ovum.

It is also known that the only venom fluid in a vampire that can poison a human is the kind in their saliva

this info is on stephenies website

I have no idea.Lolz

I´m pretty sure it has something to do with the Venom!

i don't understand why do u need blood to hv sex?