Do you think it is wrong to fall out of love in a relationship? Not really, if you are really not attracted to your partner anymore, why lead em on? However, if the couple is married and just want to throw their hands up and call it quits, then yes. I think of marriage as a permanent arrangement, and only in extreme cases, e.g. infidelity, abuse, etc. might warrant a couple splitting. Especially if there is a third party? Maybe. I always think that if there is a third person involved, don't break up with one for the other, give yourself time to think it over first before you make a decision who to be with. Do you think it is best to end the relationship if you fall for someone else? As above. Shall we follow our heart in love or follow our brains? Both. I'd most likely follow my brain alil more than my heart though.
^ I share your sentiment on the 3rd party question. However, if the affection is more than infatuation, I just don't think it's fair to either party to be led on...