if you even have the urge to want to talk to, see, or hook up with other people, then it means that you arent really serious or in love with your significant other. of course you dont want to lose them completely, especially if they have been in your life for a long time, but its just because you are comfortable and dont wanna lose that sense of always 'having someone' to fall back on. if you wanted to pursue other options, you should let them know so that they arent being strung along. it isnt fair to either of you.