That’s not a really good thing and an insult to the girl that you’ve been with.. 
I agree with this.
I say the following not as criticism, but as my different and personal perspective that may be helpful to you-
Having sex with someone who you don’t love is profoundly damaging to self-esteem, and therefore totally distorts the whole picture of love. If you can’t commit seriously to someone after having sex with them for a while you should stop. Continuing to have sex isn’t ‘rejecting puritan morality’, it’s accepting that immediate, perceptual passions take precedence over deep rational and emotional values.
I’m saying that, without feeling guilty, try and consider the effect your relationship and casual sex has on your self-esteem and your view of her - taking into account the possibility that perhaps it damages both.
That being said, my personal opinion is that sex itself isn’t really as bad as it is to keep having sex once you have a good emotional grasp of what it means to you. If you’re having sex 2-3 times a week, you’re not best friends, period. You have created a deep emotional relationship, without perhaps a good rational foundation. That is, without fully loving her. I guess I’m saying you should think about what point you need to stop.
Fantasizing about other women is fine - I’m not convinced that the animal man is meant to be monogamous. Historically, there’s been a fair amount of fooling around as well as rape in our species, even with rigid social architecture governing sexuality. I’m not convinced either that modern society, nor the Objectvist community have magic bullet answers to the question of monogamy. It’s a tough thing to think about and seems like the core of your problem.
My less personal analysis of your situation is that it seems like you actually really like this girl, and even more so that she likes you. I apologize to women, but their psychology is mystifying to me, and I am often convinced that generally men need to take an emotional lead in relationships. Given that, it seems that your conflict is reconciling your honest and true love for a woman with other sexual desires. You need to decide and lead, and not let her tears trap you. What you decide for your interest as long as you’re honest, clear, and consistent with her, is what you need to consider.
I can’t imagine a woman (or a man, but especially a woman) being all that okay with their man chasing after other women. But as a man, I understand how natural that might seem. That difficulty constrains me to offer only a couple specific and again personal conclusions:
-if you’re living with someone you’ve had sex with and would likely do so again, you can’t expect to pursue others unless your willing to live somewhere else. Living with someone means sharing life with them, even with ‘ostracizion’. If you want to break-up, move out.
-if you love the girl, and breaking up is too hard, then stay exclusive. If you meet the twin conditions of truly rationally loving her, and having the capacity to put your rational feelings before your sexual impulses, then you benefit most by staying exclusive.
-if you don’t love her enough to not want to seek love elsewhere, or your sex drive is too strong, move on! There are a lot of fish in the sea as they say, and you should only stick with one because at that moment they’re ‘the one’. Don’t be afraid to give up something you’ll miss if you can’t keep it under honest conditions. It isn’t wrong to break up with someone, and leave them behind, if you seek others. It is wrong to stay with someone out of fear and emotional uncertainty, when you shouldn’t.
I’m of the school that thinks that men and women ought not be friends the same way that men and men or women and women are. A man and women can be brother/sister, colleagues, lovers, potential lovers/courters, but not friends. This isn’t an absolute, but generally I think that’s how it is.