If the only two living humans are gay but want to propagate the human species, sperm donation seems like a sensible option.
I’m not an expert by any means, but it seems like fantasizing might help the donation process.
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"Les grandes personnes ne comprennent jamias rien toutes seules, et c'est fatigant, pour les enfants, de toujours et toujours leur donner des explications." - Le Petit Prince