Ok...well...we all know lust is a natural human response, so I don't think anyone should be condemned for having lust towards someone other than their partner. But I do believe in the following:
1) Looking is one thing, as is flirting, but cheating is wrong per se, as it's disrespectful and hurtful. It's just plain wrong. No ifs, no buts, if you love someone, you don't cheat.
2) If you do fancy someone else and see no future for your current relationship, have the decency to break it off first.
3) If you haven't had the decency to do so, or had a moment of weakness where you gave into temptation (and as humans, we're all prone to mistakes so let's not be too harsh here...) at least have the decency to THEN be honest about it with your partner.
4) Lying about it does not make it hurt any less, as eventually the truth will out and having a partner hate you is far better for THEM than having them become so distrustful because of the lies that their future relationships are ruined. Sod what's easy for you for a change, do the right thing by your partner, especially if you claim to love them.
5) For me it's only morally bad for the person who has the partner, not the other party...they are, in my view, under no moral obligation to not go with a person who has a partner...it's the one with the commitment who is under an obligation to do the right thing. Perhaps it's a little selfish and disrespectful to the person's partner, but sod it, it's a dog-eat-dog world and we all have to be a little selfish when it comes to third parties who we don't really give a rat's arse about.
(This is sort of a rushed post so if I think of anything else, I'll edit it)