The Succubus receives nutrition like an embryo in a mother's womb. The inner walls of her vagina are lined with soft tissue that is supplied with large amounts of blood, like a placenta.
The male erection is achieved by increased blood flow to the penis, so the energy-rich blood of the victim comes into contact with the feeding-organ of the Succubus.
To ensure sufficient feeding time, the feeding organ excretes a cocktail of psychoactive drugs that increase blood pressure and heart rate and dilate blood vessels (it's basically a natural Viagra), while enhancing the victim's experience and making them addicted to the Succubus. Ingesting these drugs before intercourse increases the effects, so the Succubus initiates more adventurous forms of foreplay.
These drugs cause the victim's body to maintain an erection even after the point of physical exhaustion and release even more nutrients into the blood stream. The extremely increased heart rate and blood pressure may lead to cardiac arrest in older victims. Younger victims who can feed the Succubus for a longer time get addicted to her drug cocktail and seek her out again and again until their bodies are too exhausted to recover. If they are physically kept from seeking out the Succubus, the withdrawal makes them go insane.
We're talking about a fantasy creature here. Although it might be possible for her to obtain a minimal amount of nutrients, it won't keep a fully grown Succubus alive, least of all during strenuous activities.
- The placenta of a woman is just enough to keep the fetus alive at the moment of birth. The size of the placenta limits the growth of the baby.
- The placenta is literally made for the exchange of nutrients between two blood streams that never mix. The human penis is not.
- Inside the placenta there is a relatively big area where the blood streams of the mother and child touch. A penis has a much smaller skin area and most of it is probably still too thick to allow the exchange of nutrients
Carbohydrate is transported to the fetus as glucose which is taken up from the maternal plasma by the GLUT 1 transporter and transported to the fetus by facilitative diffusion according to concentration-dependent kinetics. Protein is transported to the fetus as amino acids by specific amino acid transporter proteins. Placental lipid transport to the fetus involves direct transporter mediated transfer of certain fatty acids as well as lipid uptake from lipoproteins, metabolic alteration in the placenta, and release into the fetal plasma. (source)
- All these transporters are missing in the victim's body, so the actual exchange of nutrients is impossible without injecting them first into the victim.
So if you really, really want to, you can give your fantasy creature a fantasy feeding organ and handwave the problems away. If you want to be a little more realistic, you have to go for a much bigger surface area and much thinner skin to leech nutrients from a victim. Like the intestine, which is gross... but more plausible than the penis.