It doesn't make you very nice. You're selfish and uncaring about people not close to you - a lot of us are in various ways, it doesn't make us horrible people through and through. There isn't one person here who is 'just a nice person' without any unpleasant aspects to them, or their behaviour. Whether or not they're shagging men in relationships.
I completely understand why these mens partners harbour a lot of anger to the other woman - sexual jealousy is very real, competition for a mate that is valued etc - it all makes very logical sense. That being said, I struggle with the idea that it's up to the morals of another woman, to keep someone's man from cheating on her and causing a load of pain and anguish, and essentially, override the morals of the man with a partner and/or family. If your mans a cheater, he's a cheater, regardless who with, and most people would never know he was selfish and (in the eyes of the wife) immoral in this manner, unless another woman was willing to engage with him in some way. Ignorance is bliss, it seems.
What you're doing makes logical sense for you, it just isn't very moral in todays society, and you'll gain a lot of enemies this way. I hate to say it, hence the name change, but there's a reason why so many women and men are willing to engage with a person committed to someone else, and it doesn't always boil down to low self esteem at all. It can be the case in some circumstances, certainly, but actually it's often just a case of decompartmentalising and doing what suits you.
For women who don't want a man who is going to try and tie her down, wants the positive aspects to a relationship without all the bullshit of it, firing out his offspring, picking his minging socks off the floor, nagging him to do basic shit he should be doing, then an attached man does indeed, unfortunately, make logical sense. A great number of them treat their affair partners very well, financially, romantically, emotionally, acts of service etc, often to the point of treating them better in these ways, than the wife. Is it socially right of him? Nah, absolutely not. But plenty of people are quite happy to have these positive aspects, and it suits them fine because they don't want to deal with the negatives.
There's a reason for this. Most wives prefer to label the other woman as insecure with low self esteem, it makes her feel better.