Like I said on the Leyton vs. Brucas spot, I never understood why this was an issue. He wasn't dating either girl at the time...matter of fact, neither girl was speaking with him. Why shouldn't he go out and meet other people? I'm not saying he should have slept with some random stranger, but when he did it wasn't betraying either girl.

^ I agree, even if Nikki was a bitch, he was single at the time

Sorry in advance for my long response...

I never really thought of Lucas sleeping with Nikki as an act of betrayal toward Brooke... but I think that's mostly because after fooling around with Peyton while dating Brooke- I don't think Brooke really cared what else he did. If she didn't know about Peyton, it would probably crush her to know he slept with someone while she was trying to make things better between them. But since she knew about Peyton- I don't think anything could have hurt her more than that. So maybe Lucas did betray Brooke... but considering he had been hooking up with Peyton- I don't think it was surprising to Brooke or the audience that Lucas would do that.

I did however think he betrayed Peyton, or at least his feelings for Peyton-- and at the time I did find that surprising (in retrospect I guess it's not surprising but when I first saw season 1 it was surprising to me). He goes from one minute saying Peyton's the only one he can think about, to the next minute sleeping with a stranger. Just because Peyton said they couldn't be together right then doesn't mean his feelings should just fade away. It wasn't as if this was a month or two later, this was just a day (or a couple of days) after Lucas told Peyton he can't get her out of his head & wants to be with her, and Peyton said she needed to be a good friend to Brooke & not see Lucas.

Lucas says in 1x15 that he wishes they could just fast forward the next 6 months so he could be with Peyton... which obviously demonstrates that he knew it would be a while before he really would get to be with her. So it didn't really matter that Peyton said they couldn't be together at the end of the episode since either way they couldn't be together right then. But that means he should go sleep with someone else?

And in 3x09 when Brooke comes to apologize to Lucas and she comments on how he slept with Nikki- his reply is "I didn't have feelings for you then." So he was able to sleep with Nikki cause he didn't have feelings for Brooke, but what about Peyton? Did he not have feelings for Peyton then either and that's why he was able to sleep with Nikki? Seems unlikely since he had pretty strong feelings the episode before. So then that justification doesn't work with Peyton.

But then again- I guess it's not surprising in retrospect considering the day Lucas & Peyton broke up in season 5- he kissed Brooke (and possibly would have slept with her as well if Brooke hadn't pushed him away). So I guess Lucas likes getting physical when his heart is breaking.

I think people focus so much on what's permitted or not permitted in a relationship- that they forget that the whole reason behind those "rules" is that you don't want to hurt the person you care about. Brooke & Lucas weren't exclusive when she slept with Chris Keller- but heck yes do I think she betrayed Lucas. Nathan & Peyton weren't together when Nathan slept with Brooke... but I definitely do think both Nathan & Brooke betrayed Peyton. There's no rule about keeping everything in a relationship private, but I totally think that Julian betrayed Brooke when he told Alex that Brooke couldn't have children.
You can do things that aren't exactly qualifed as "cheating" that really betray the person you care about b/c you're doing something wreckless that hurts them. In my opinion betraying someone is not just about kissing or sleeping with someone else when you are in an exclusive monogamous relationship--- it's more about being disrespectful and wreckless with the heart of a person you care about. And I definitely think that's what Lucas did.

It was a betrayl, technically but it was a dumb fail. You cheated on Brooke with Peyton and then it all got twisted and what do you do? You sleep with a stranger. Great, Lucas..just great.

So if he said he wants to be with Peyton and told her that as well and she turned him down in a way , he wasn't allowed to have sex ?

Even if he was in a commited relationship, everyone should expect it from Lucas.

I can see why they were pissed but they both over reacted, it's not like her actually liked Nikki. Besides he was kinda drunk too.