[QUOTE=hawksgirl]
I assume you’ll pretend to be extremely dense and fail to see the differences between the two scenarios, and call everyone in the thread a hypocrite. Am I close?
[/QUOTE]
Dingdingding! We have a winnah! 
The difference between what the guy in the OP did and watching porn (or strippers) is that in the OP scenario, the guy and the neighbor participated in a mutually consensual sex act. Granted, it wasn’t the customary sex act, but it was clearly spelled out by the neighbor as her sexual act of choice. Participating in a consensual sex act with a person outside the relationship (absent any agreements to the contrary between one and one’s SO) is flat-out infidelity.
Rubbing one out to porn isn’t a sex act with another person. By definition, pretty much. I don’t know how to express it more clearly, but the stars in a porn movie aren’t involved in the sex act with the person masturbating. Both the guy in the OP and the neighbor were involved in a sex act. He was watching, she was doing - but his presence was reportedly necessary.
With the strippers scenario, see the reasoning for why porn is different.
Keep in mind, that with some partners, either porn or strippers or both could be considered as participating in a sex act with someone outside the relationship. Mileage varies on that point - but if you’re in a relationship with someone who feels that way about it, and you know it, then indulging in either porn or strippers is cheating. Everyone gets to decide for themself what is and is not sexual behavior they’re comfortable with in their own relationships and act accordingly. If porn and/or strippers aren’t okay, and you let your partner know, then that is precisely as valid as a decision that anal sex isn’t okay - or that you’re not into bondage. It behooves the partner of someone with stated preferences to either agree to abide by them, or gracefully bow out and seek out a relationship more to their personal taste. To agree to abide (even by implication, by staying in a relationship knowing what your partner’s expectations are) and then fail to actually stay within the known limits of the relationship is cheating.
Hell, I’d side with the girlfriend based on his behavior outside the watching the neighbor get off alone. He made several really, really questionable judgement calls IMHO - he was a) drunk, b) flirting, c) agreed to go inside the neighbors apartment for more of same, d) failed to disengage himself when it was pretty clear the situation was moving Due Sex, and then e) (and this is the killer for me) treated his girlfriend’s poor reaction to his poor behavior as though she were being totally irrational about it. Her being pissed in this situation isn’t an overreaction - as to whether or not dumping him is, I’d have to know a lot more about their relationship than was presented in the OP to adequately judge.