The Wilhelm Scream, a stock sound used over many years in Hollywood action films, became an insider joke for sound engineers because it was an exaggerated comical sound. It fit the genre because it was silly and fun.
I suggest a much more harrowing thing be done for the grunt. An involuntary sound, it is primal, a release of terror, pain or pleasure. I will suggest one for each category:
Stevie (John Savage) unleashes a ghastly, breathy grunt waiting in fear for his turn in Russian Roulette in Cimino’s The Deerhunter.
Roy Batty (Rutger Hauer) panicked yet controlled release while piercing his hand in Scott’s Blade Runner haunts me to this day as does the death grunt of The Ugly (Lee Van Cleef) from Leone’s The Good, The Bad and The Ugly.
The pleasure grunt is a little more broad, but a simple porn grunt search might suffice. Perhaps try Tanner Mayes?