Hello everyone. Thank you for your answers. I would like to add more details to my question.
I'm not sure if I would break up with someone addicted to pornography (nevertheless holding that relationship would cause me a lot of pain), but this is a very important point. I wouldn't probably do it if he/she told me that. If I am the one who discover it, I would maybe break up.
I will leave a relationship in which there is unfaithfulness or abuse.
But I would like to focus more in the pain topic. When you start dating someone, you feel that person is the most amazing guy/girl you've ever met. You feel that he/she is perfect in every aspects of a human being (beautiful, intelligent... We all know that part of the story, let's skip it.) So, the reason you start dating he/she is that you would like to marry him/her.
But always at some point of the relationship, you realize that person is not perfect (of course he/she isn't! Nobody is!) That person has just as many flaws as you have. And, what it's worse, those flaws annoy you. I see that as normal, and you have to pass through that, whether you like it or not.
I do agree when some of you guys said don't date someone with the hope of changing him/her. So, I assume those flaws are going to be there for ever (although there is always room for hope). And they will annoy me as much as they are annoying me right now.
However, another part of me likes those flaws and enjoys seeing that person knowing he/she has them and also seeing the effort that person is making to fight them despite the fact there are going to remain there forever. And I feel very connected to that person just because he/she fights against his/her flaws as I do the same with my own ones.
Another important thing, I talked about flaws, but that person also has his/her virtues. And they look for me as good as they looked for me the day I knew I was in love.
We'll I was getting romantic up there, I'm sorry. The question: "how do I make abalance between flaws and virtues in order to decide I want that relationship?"
And, if you were patient and read this far, I have another question: "how do I distinguish between a bad moment to make a decision about the relationship from a good one?" For example: when he/she is in holidays for (let's say) 15 days, I think a lot about him/her and about our relationship. Do you think I should? Sometimes my happiness and emotional state depends of things like he/she texting me that day and the tenderness of the message. As you can deduce here, I'm in a relationship in a very early stage.