It's entire legal to cheat on love. Maybe it's easier on your gf but not wife. Because the end result is that if you cheat on your wife and you get caught, a divorce follows and she will squeeze a big helluva amount of money from your bank account. So, you might not want to do that if you know that you'll be caught. I'm sure rich men/most men know better.
Morally, it's always wrong to cheat. Our society does not condone cheating and it's definitely not accepted in our ways of life because we cared too much what the society thinks of us! It's also in the common understanding that if you love someone, you do not want to hurt him/her. If you cheat on your partner, it automatically means you don't love her. That's what majority of the people thinks anyway.
This is entirely my own opinion. Like I said, love and sex is two different entities. I might be physically attracted to someone but that doesn't mean I love him. Love is a big word. You do not say you love a person if you're not emotionally and physically attached to him/her. Personally, I think people cheat because they find the thrills in cheating. It's fun if you don't get caught! If you know that you'll be caught doing this, then what's the point of cheating? It's just like if you love Cadbury chocolates so much and one day, someone offered you an Amedei and tells you that this is the best chocolate in the world, would you want to try it? You might take a bite and then after that you go back to eating Cadbury and life goes on. But of course, you can always switch to Amedei and dump Cadbury.
So, personally, to be legally & morally correct and yet be happily attached in a relationship : My advice to you who cheats, getting caught with your pants down is the last thing you would ever want it to happen!