i used to see different girls at the same time regularly so i used to suscribe to the cassin logic. Then one time i started seeing this girl, really casual just someone i met at a bar, we had sex on the first night and we met up a few times for more of the same, then she asked if she could meet me on the thursday but i was busy so we arranged to meet on friday where we had sex again and it was apparent we had something a bit deeper going on. Anyway we wound up in a relationship and went out for over a year. I found out toward the end that on that thursday night, when she couldnt meet with me, she met up with another guy and had sex with him. So i had sex with her on less than 24 hours after she had been taking it from someone else.
Now by the cassin logic thats perfectly sound, which to be fair, i still agree, it is, we werent committed to each other yet, but i would be a fool if i said it didnt really sting when i found out later on. My memories of meeting her and having what i felt was a real connection from the very first moment were somewhat sullied by this fellows errant c*ck in the proceedings!
Since then my principle has been to take it one at a time just in case something develops, not out of any real moralistic view as your not really doing anything wrong when you rationalise it, just keeping things simple tends to make life easier and keeps things pure!