Does orgasm cause a dopamine crash?
To answer your last question, I very much doubt it's true in general. It does have unplesant effects in some people though. Form a popsci account


  He [a psychoanalyst] was puzzled about a 24-year-old man whom he viewed as psychiatrically healthy except for intense depression that lasted for several hours after sex. [...] Could it be that some patients have particularly strong rebound activity in the amygdala after orgasm that makes them feel bad?


And this post-coital dysphoria can happen to women too, for which there is some prevalence data:


  Forty‐six percent of respondents reported experiencing PCD symptoms at least once in their lifetime with 5.1% experiencing PCD symptoms a few times within the past 4 weeks.




"No sex before game day" was a tenet in some sports psychology/physiology corners, but more recent research failed to find significant differences of sex acts on mental performance... at least in (healthy) athletes. A 2016 review notes


  Despite the major differences found in the recovery phase, where higher values of HR [heart rate] were found 2 h after sexual intercourse, no significant differences were found in workload achieved and in mental concentration of the athletes. 


And, yeah, I know sex doesn't necessarily mean orgasm, but it seems close enough to mention.