QiQi riala in the encounter after you kill tal and save aka said something abot the line: "you plan to sex-starving you to death? with your willpower you probably can do that"
and resistance to sex magic and willpower aren't the same. it's probable simon reach his high resistance to sex magic (and immunity to sex status) through self discipline, but they are two separate things.
another point through the uniquies of simon, mestan had said to simon changed when given the power, he was a basic soldier before that, without any potential, but when he gained the shard, a true source of power, he became something far greater fo the banal sum of his old ability + new power.
alonon was able to foresee the reunion between simon and wendis, their need of the true stone for rebuild her body and probably something else we don't know it yet, and he was a 'normal' person. it's not a stretch think a goddess wth centuries of experience will be able to foresee at least the potential inside simon, if someone can place him in the right place to be able to blossom his full potential.