No really it's a douchebag move that will eventually backfire on you. Lets assume you start hooking up with this new girl and begin cheating. You run into two real problems.
1. If the new gal doesn't know about your current GF and they both find out you're cheating, and they will. You are screwed, because neither of them will tolerate a sack of crap for a BF. One thing all women are famous for is talking to each other and spreading around information. Both your GF and new gal will tell everyone about your cheating ways, and their friends who might have been interested will be turned off to you.
2. Trust issues. Let's say you really care for this new girl, and she knows that you cheated on your EX with her before you broke it off and started going out. Your relationship with begin under an act of infidelity. Eventually she will wonder that since you were willing to cheat on your EX before you started a new relationship, how does she know that you are not doing it right now. If not that she will know you are capable of cheating and it doesn't bother you. How much do you think she is really going to trust you.
Now after 9 months you've been in an unhappy relationship from the way you make it sound. But instead of being a man and saying its over and breaking up you are looking for an easy way out. So you decide to cheat, and knowing you will get caught, she will then break up with you. Instead of you having to do the work of bringing up the fact you want to end it. Like I said earlier this right out of a woman's playbook. They cheat to get caught and leave you no alternative to end the relationship. Just be a man and end it with diginity. You protect your reputation by not being labeled a douchebag and cheater. Even if its to hard to break up with your GF the very least you can do is get her to agree to see other people and then decide what happens between you.
-Grey Fox
Am I losing women? where were they when I was single ?