Before You Embark On An Affair....I have experience, not that I'm proud, because I'm not but if I can use it to save you some hassles and heartache, I'm all for it.
First, if your intended partner is also married, that is a threat to yourself, your marriage (if you have no immediate plans to leave), your employment and possibly more. SOME married people who are habitual cheaters a well versed in getting what they want. They may tell you their spouse is an ogre or some other equally colorful beast and that may be the furtherest thing from the truth.
I was single when I dated a man I thought was getting a divorce (lie number one). We dated approximately a month or two. His wife caught him in a lie about where he was and he confessed everything to his wife, including pinning his money spending on me (lie two). Sooooo his wife calls me at one am and threatens me if I hang up she will come to my job and make a huge scene. I answer all her questions, and several phone calls later (and uncovering more lies) I got her to understand that I was not the problem, her husband was. Years later she saw me in public and literally physically attacked me.
The lesson? Don't embark on an affair with someone you don't know - better a life long friend then someone you don't know so well. You have no idea what sort of person they are married to or really? what sort of person they really are - they could stalk you and this of course would really interfere with your own marriage.
Second - if you are with someone who treats you very well (within the limited time, energy and money that affairs dictate). You will resent your spouse even more - the sexual frustration that fuels your rage will have rocket fuel poured on in massive quantities. And frankly? You will most likely scream at the unfairness of your choices, etc.
Each person must make their own choices - but if you are going to choose to have an affair, be prepared for every possible scenario - especially if you are in a situation where something requires you to stay with your current spouse. A lover (particularly a married one) may not want or be able to support you and/or your children.
I suppose it can be done well and in a balanced way, but we did pick a person who doesn't care enough to provide us with the one thing we can't get anyone else. If you are going to pick someone, improve your picking skills.