how do you end a relationship without causing damage?

Depends on the situation.

If you have kids, it's a nightmare that requires great care to negotiate it so that they don't suffer. It means being willing to set aside all vindictive thoughts and judgements about the ex in order to cooperate in equal parenting for the kids' well-being.

But if you don't have kids you can walk out of a relationship anytime without causing any damage.
How the ex responds to losing you is their responsibility, not yours.
The best way is to do all the preparations beforehand,
then be up-front and honest about it when you tell her, and then just go.
I think it's better to make a clean break and not have any contact afterwards.
It makes it easier for the other to accept the reality, grieve, and move on.

However, there are situations in which the above does not apply.
If you've always had a good friendship, then after the relationship is over, the friendship can continue.
Or, if the ex is a control freak, then leaving can put you in danger and you need to move house, change jobs, get a new phone, and ensure that none of your family or mutual friends will give away your whereabouts, or cut off from them as well.

If the potential-ex is a suicidal, a self-harmer, an addict or has some other form of high risk - you are NOT responsible for their reactions or actions.
These behaviours are psychotic and they are how they try to manipulate others around them. If they are serious about getting help, they can go to professionals.

Just because something hurts doesn’t mean there’s damage. To lead someone on, to allow them to believe you feel the same way about them, that they do about you is damaging. To allow them to believe nothing is wrong when you’re trying to figure a way out is cruel. The best way to end a relationship is with empathy and honesty. Don’t tell them there’s a chance if there isn’t. Don’t say unkind things, that aren’t necessary. Be concise and quick but not heartless.

tell them you are gay

As gently and sensitively as possible. It's bound to cause upset though - there is no getting around that but if you can part on friendly terms that will help a lot.

You don’t. Unless both of you are willing.

That’s what I’m trying to figure out especially when you’re with someone that could easily turn like a pit bull on heat.

Just disappear, if you can.

Well... you could say that I'm expert in that...

You can't. Not if you still care about each other.

One last passionate kiss.

Is that even possible?

Just accept the damage