'Til My Boyfriend Do Us Apart

One of my really exotic friends said that she is going to get married soon, it was a real blow when she continued with the sentence "to my cousin :D!", you see, the story wont be as shocking as it is to you guys "Muslims and Kuwaitis" because in our culture, marriage to cousins is a custom thing really, a ritual one might say, but to you Christian friends, it’s a major taboo, well, I am not speaking Christian wise, the girl has a boyfriend, and dare I say, they are sexually active, which means in our culture, they're meant to be burned on a stick, any way, so she is saying that she will get married to her cousin, though she is in love with her boyfriend, then she completes with the idea that if her boyfriend really loves her he should stop the marriage and force her parents to give her hand to him to marry.
Now that is not the issue here, still it is not people, the issue is could you stay with someone who's married? Wither you love the bastard or not, will you stick around?
Frankly, if I was in love for real I would, but how would it make me feel if some one is backstabbing me and screwing behind my back, and still late at night when she comes to me she tells me she loves me, and another question regarding the married person if he said that he could love two, could any one really, love two?
Now this topic maybe as scrambled as heck, but hey, some of my questions are complicated, and ya'll complained about that many times, still I have to ask you guys…
Can anyone love two?
Can you backstab your loved ones and cheat?
Can you date/love a married person?
Can you lie, saying I love you, everyday, to a person you're married to, and the truth is that you don’t?
Can you I mean seriously, can do it, get married though you are in love with another one?