I can not imagine only once cock and I agree with Lynn. I pick up men in the grocery store every other time I shop.I imagine that it could get boring over time which is why so many couples fantasise about or get involved in swinging or sharing.
The fact that I fuck other guys has kept our sex life exciting so no we haven't got bored with each other. But I can't imagine having to only fuck one guy for the rest of my life no matter how much I loved him or how good a lover he was.