Human males have evolved to produce more sperm when they perceive they have competition. They have sex fantasies like "gang bang". Cuckoldry is as much as a fear as it is a fantasy for them.

https://en.m.wikipedia.org/wiki/Human_sperm_competition#Sexual_arousal_and_sexual_fantasies

Moral of the story: flirt with other guys until you see that ring on your finger. Date multiple men. Make them invest and pay for dates. Even when you decide to commit to one man, don't make this man a center piece of your puzzle. Make it seem like you have other options even if you don't care about other options.