People are their own beings with their own thoughts, feelings, wants and desires. Nobody in a relationship has any obligation to stay with you and if they feel they can do better they will.
I find it rather silly to believe that pure sentimentality can keep a relationship together if someone can do better then their current partner or that infidelity won't happen if things hit a crossroads.
And understanding that I realized I have essentially 2 options. Either I can allow myself to be paranoid about whether my partner will cheat on me, eventually becoming overly controlling and turning it into a self fulfilling prophecy. Or I can accept that I have no way of stopping them even if they do cheat, that they probably will eventually and just enjoy the ride while it lasts.
This is part of the reason I feel open relationships make more sense these days, not as an excuse to sleep around, but understanding that nobody is obligated to stay or have sex with me.