.. the idea that when you have someone you can have anyone.. that's a problem, people get greedy, if youre having sex every night with your partner and then you make eye contact with a cute girl at a cafe one morning, you think 'damn I could be having sex with her every night instead!' but ITS NOT TRUE!! Odds are that you would not be having any sex for a while, and even if you were, it wouldn't be someone you'd stick with for months, maybe even years!
You are NOT missing out on loads of hot sex, you are just missing out on some occassional and mostly disappointing flirty encounters. Ok girls can probably have sex with guys more easily than guys and that's why when you're a guy it really hurts, because its a fact, its easier for girls to have sex with people, they make the decision.. but I digress, point is, mostly youre not missing out on anything by being with someone, only very occassionally. But the question mark shows up sometimes when you make eye contact, and its very alluring! Let it go. Forget it. Enjoy who you have as much as you can for as long as you can. That doesn't mean don't pash anyone, don't flirt, don't play with your sexuality... it just means don't think youre being obligated or constrained by your partner, that's an illusion, its all you.