Personally, I think lying is a part of cheating. You can lie without cheating but you can't really cheat without lying (well I guess you could but generally they go hand in hand). So with that in mind, I would say cheating is worse than lying.
If a guy cheats on me, and tells me either before or after, and is honest with me that he will continue to do so... and I stay with him anyway... then it is nobody's fault but my own. The guy is being honest, and putting the ball in my court.
However, I see lying (lying about any big issue, really) as taking away my power of choice, and trying to control me by controlling the truth. He isn't giving me the option of deciding for myself if I want to be with his faults.
Lying and cheating are close to the same thing, to me. Neither are acceptable, in my opinion. They are both a form of betrayal, and once you find out what's going on, it hurts so much knowing that you put your trust into that person and were always honest with them, and they turned around and lied to you or cheated.
I am pretty sraight forward. So lying is a huge thing for me, If I can't trust you its over. But also with cheating that would make me end it too. I wouldn't cheat on them so why should they cheat on me. Trust is the biggest thing for me and with either things happening I wouldn't trust who I was with again.
I've been lied to and cheated on and they are both unacceptable and unnecessary to do in any situation. They both show that they don't care for the other person. But they guy that lied and cheated on me, I'm still with him only because I want to give it another shot and try to move on.
My gf broke up with me today because I messed up. Last night I was drinking with friends, a guy kissed me and I kissed him back. I told my gf this morning and she's ended it and of course is extremely upset and hurt. This was the first time I've ever cheated on her and from this experience I know it would be the last. I know now, more than ever that I only want to be with her and would never jeopardize the relationship again. As for her, we've had some issues with her and a girl that she used to like that ended up becoming a friend of both of ours. I did not have any issues with them being friends unless my gf would put her before me and it bothered me because I knew that my gf used to like her. I had asked my gf and our mutual friend if anything ever happened and both of them said no. A year and a half later, my gf confessed to me that they made out twice before she and I got together. I was hurt because I don't like lying and I don't like being lied to. But my gf and I worked it out that same weekend and grew from there. As for the question of wether lying or cheating is worse? Well, I believe that both are horrible of course, but for me (and this may seem biased) lying is worse because it diminishes the trust that was there and as someone else said, is out of your control. With cheating, It happened once and I told her the truth as soon as I could, but I get that trust was broken with that too. That however can be fixed in my opinion. I believe that there are things that can be done to ensure that you are fully committed, loyal and faithful. With lying, that creates such an insecurity that you question wether the other person is lying more often. This relationship that I had with her has been a long distance (12 hours) relationship for 6 months and so maybe that plays a role in all of this. I don't know, but I'm just hoping she'll take me back.
I think lying is worse. Cheating also falls under the category of lying. With cheating, it could have been an one time thing, it could have been a mistake that won't happen again. However, with lying, it all starts with little lies and when he gets away with those, they become bigger lies.
Cheating makes lying necessary. I think the lying is actually what damages the relationship because you have to look at your partner as an adversary. I think people make a mistake by assuming monogamy. Many relationships do well with additional outside sexual contact as long as everyone is honest about it.
(I'm assuming here that you're not talking about a situation like an open relationship where in certain circumstances other partners of your partners are explicitly allowed. Given that...)
To me they're identical in that they're both deception. Lying is, obviously, lying. Cheating is breaking an agreement that the two of you made. It may be a de-facto agreement or more explicit like in a marriage, but it's still an agreement that you both (presumably) entered into in good faith.
Lying is deception, and cheating is deception plus sex. Since for me sex itself isn't a hot-button topic, I see them as being pretty much the same thing. Neither is better or worse than the other. If it's an argument like, "You lied to me!" "Yeah, but you cheated on me!" where the cheated-on partner is trying to establish the moral high ground, then I'd personally see both parties as having shown themselves to be equally untrustworthy. Don't know if that's what you're talking about though.
I'm guessing most people would probably consider cheating to be a stronger deception than simply lying, but I don't see it that way. They feel the same to me. Truth is very important to me; sex is no big deal.