It's been a long time, I thought maybe i wasn't remembering it right, so I checked this review of the movie by
Roger Ebert in 1976:
"But the moment comes, as I suppose it has to, when Billy Joe reveals his secret to Bobbie Lee and then jumps off the bridge (mercifully, we are not treated to a shot of the jump itself). His reason - that while drunk for the first time he has just had a homosexual relationship ("a sin before God and man") - would, I suppose, have been a compelling one for an uncertain adolescent, circa 1953. What's more, he then finds himself impotent with Bobbie Lee."
I can see reasons why Bobbie Gentry didn't like the movie, but I thought this seemed to fit the song and it was done with sensitivity, showing how much young gays used to suffer not that long ago.