You know, it's not easy to be open and honest about a non-monogamy choice, because we have been so heavily socialized to believe it's wrong and bad and harmful to make a choice other than monogamy.
I'm not surprised that people go into LLTMBRs with a "we'll be together forever!" attitude, because we're taught that that's normal, and anything else is abnormal (wrong, bad and harmful). In so many ways, from so many directions, we are bombarded with these messages that you are supposed to get married, stay married, be sexually faithful, and all the other "debts" about relationships or there is something wrong with you. And then there is so much blame and shame when you "fail."
In this sort of atmosphere, it doesn't surprise me that people would lie to their partner and to themselves about their monogamy -- not necessarily out of maliciousness, but just trying to cope with the demands of being what you think you're supposed to be.
Some of the posters here are saying, "well, it's okay if you agree ahead of time that you'll have an open relationship, but otherwise you're a scoundrel if you cheat." I think that sort of feeds into your own feelings of what a relationship *should* be, and sets you up for limitations on freedom, love, and connection. All of which is not to say that it's wrong to make a commitment of sexual fidelity. Just that: why not practice letting go of making each other wrong when sexual infidelity rears it's head (so to speak
)? Pretty much everyone at least thinks about sex with persons other than their partner, which leads me to believe that we're not inherently monogamous creatures. Again, it might be a good idea for you to make a sexual commitment anyway -- but recognize that you're swimming upstream when you do. That might be a really great game or challenge for you, but remember: there is no failure, only feedback.
Under the circumstances of all this pressure, doesn't it make sense to cut your partner (and yourself!) a little slack?