No way. Not with my man. But that's because we're we made a commitment to one another and if you can't trust the one you're with, then you have nothing. Trust in a relationship is everything and it must go both ways.

To keep someone on the side as a "maybe" is not only disrespectful to your SO, it's disrespectful to the side-dish (for lack of a better term.) Leading someone on because you just don't know what you want, is one thing (though not good, it happens sometimes). Leading someone on deliberately so that you have a fallback is hurtful. Messing with a person's emotions just because you can will only hurt people, including you. Eventually you will lose both. You may be alone then. You will lose friends, reputation, and perhaps even self-respect.

So unless these two are interested in a plural relationship, you must decide. You must figure out what you want from your life. Then you can be in a healthy romantic relationship. You have to be able to thrive on your own, then you will be able to find a fulfilling relationship. It's the only way, or you will always be unhappy no matter who you are with.

That said, while I am very happily married for many years, i still have good friends of the opposite sex who at one time expressed interest in me. I am clear where I stand in the friendship about my stable marriage and absolutely transparent with ALL interactions with other men to my husband. I would not do anything that might even make him feel threatened, because he never could be. He is mine, I am his. Or we would not be together.