Do you believe, "all is fair in love and war?"

No definitely not.
A long time ago, in the medieval ages, they had some concept called a "just war".
It meant a situation where you had to fight someone, but had to do so in a "right way". For example, consider a duel: You have to talk to your opponent first, introduce yourself, state the reason why you're having qualms with them, give them an option to find non-violent ways to resolve the conflict, and only if that fails, you start the physical fighting. After the opponent is defeated (for example by giving up), fighting had to stop and the matter had to be considered settled. it was not acceptable to blindside an opponent, play dirty, like attacking them from behind. and stuff like that.
something similar applied to larger battles as well, btw.

As far as I'm concerned it is a phrase you use as a joke when backstabbing someone in a game. I don't imagine people use it seriously but I've seen dumber

In war, yes. Warzones are the ultimate immoral area where you can do whatever you want, and is encouraged to do evil acts. That is why I am in general anti-war.
Love? Absolutely not. Pretty sure everyone agrees that cheating on people is not fair in love.

Uh, no, that's the sort of shit that someone wants to do war crimes would say.

Let me phrase it this way: If someone I was dating said this to me, I would run as fast as I can. And my first concern wouldn't be that they might commit a war crime somewhere.
- To be honest I was mostly curious about the love part. Obviously then Geneva convention exists but I've never been a fan of the, "I like the hunt" types.

Absolutely not. We need to have decency in war whenever possible eg not targeting children
- The war part usually isnt meant a literal war against nations. At least i haven't heard it that way

No. That's what laws are for. And I hope that the current struggles will lead to more laws to be enforced in war in the future.

No. Pretty sure the origin of that quote is a Shakespeare villain.

In love, no absolutely not. That sounds like a justification for rape, spousal abuse, stalking, harassment, cheating, and other kinds of shitty behavior where it is neither expected nor wanted.
In war, yes. I think that war is one of the most horrific things one nation can perpetrate on another and should be exactly that.. If your goal, as a nation, is anything less than the genocide of your target nation's people, the salting of their land, and the complete eradication of their culture, then your issues can and should be solved diplomatically.
And if that is your goal, fuckin' do it.
- I think that phrase is meant in the figurative "war" not the literal nations going to war
  
  That makes more sense. So, personal conflict rather than national conflict?
  In my present state of moderate intoxication, I'd say something similar to my above point, but with the opposite statement regarding fairness. If you are unable to resolve your differences via civil discussion, then settle it with a duel with strict rules as follows.
  The rules of which should be:
  the field should be cleared of bystanders, excepting an impartial referee
  The only weapons permitted should be flintlock, smooth bore pistols
  Shooting should occur at high noon (or whatever time would give both parties equal visual impairment due to the sun)
  Parties should stand twenty paces apart
  Parties should fire one round each and then move an additional ten paces apart
  Repeat step 4 until at least one party is dead