So either they've all been conspiring to lead him on, or the people who know Diana most intimately can't tell the difference between her in love and her being a manipulative jerk.
Well, they've never seen her in love (that I know of, at least), so maybe they're just not used to it.
plus Adam-and-Eving is about the most ineffective and silly way to repopulate Themyscira I can think of.
Oh yeah? What about running around yodeling while cutting the heads off chickens?