Just found this graphic. It's great because this is what I imagine might happen during a porn session where you edge (use porn without climaxing) for a while. Note that all the other graphs are taken from rat research, where a probe was inserted into the nucleus accumbens of the reward circuitry. Please know that no one has measured dopamine levels in human brains during sex or porn viewing.
One problem: the graph has sex at 200% but porn at 250%, indicating that porn viewing elevates dopamine beyond great sex. I don't think that is the case. You may be thinking "I get a better erection with porn", or "Porn turns me on more than than real people". If this is the case, porn use now releases more dopamine than real sex. However, its neuroplastic changes such as sexual conditioning and/or addiction-related brain changes that have created a discrepancy. For example, if the best sex you could possibly ever have raises dopamine 200%, we can envision 200% as analogous to the speed of light: it can't be topped by natural rewards (including porn use). If you suffer porn-induced ED or have conditioned your sexual arousal to Internet porn, sex may only elevate dopamine 140%, while porn could jack it up to 170%. Your conditioned brain now perceives porn as greater than sex, but you haven't broken the natural limit (200%). These percentages are for example only.