This thread's gonna get closed soon I reckon.
To toss my hat into the ring:
It takes a far stronger person to remain cool and calm. Nix is making the most sense here frankly. The raw emotions one would feel are expressed by converse_uprise. Frankly if you can channel those emotions into something else or better then it would help the situation that you find yourself in (this for us, hypothetical one which is stoking an argument).
Girlfriend who you are going to marry and means everything to you means that if you see your mate pulling her you shove him off tell him to **** off and never see him again. You have stern words with the girl as despite having drinks involved it shows that something is intrinsically and subconsciously not working in the existing relationship.
The fact someone cheats is never entirely down to the booze. It's usually because there's something wrong and beating someone up in front of her will probably exacerbate said problem and will probably traumatise her by you showing a side of you that she doesn't want to see (unless she likes being fought over

): that's you turning into a great big chimp and giving someone a drunk kicking. Probably landing you in the drunk tank and then getting something on your record. Kiss goodbye to getting a top job. Not that it would really be open to someone with the attitude that a huge beating serves any purpose. Because any physical confrontation (I'm imagining that this isn't happening in a nice bar in a nice district of london, but in a grime hole serving cheap booze that you couldn't resist and got you hammered in the first instance, the kind of place you see on the Police shows they have or in some documentary about binge Britain.) is not the answer.
This is what is more wrong: thuggery and debauchery and moral bankruptcy on our streets. Not men who can't "stand up for themselves".
Frankly, pulling a mate's bird is not on. Don't do it. It will lead to consequences.
Beating the crap out of someone isn't really the best solution. It could lead to serious consequences (I'm thinking a resentful ABH charge or worse here...).
Bird pulling a mate is the worst. It has the worst consequences and also the worst implications. Furthermore it is the hardest to deal with and to react to, hence the simplest action known to man: ug-bash, alpha-dominance.
I think it's important to stand up for yourself and protect what's yours. But equally I think that there is a line, and that the brain should flex (even for a split second to check no witnesses or bouncers or pigs before you mete out justice) before the muscles.