Goldstein A, Hansteen R W
Arch Gen Psychiatry. 1977 Oct;34(10):1179-80. doi: 10.1001/archpsyc.1977.01770220061006.
In a double-blind study, intravenous injection of naloxone, at a dose sufficient to saturate the opiate (endorphin) receptors, had no effect on sexual arousal, penile erection, ejaculation, or orgasm in a human subject. Apparently, endorphins are not involved in these phenomena.