I've turned many women over the years as I had no interest in them. I rarely have sex with women I don't know. You've really got to be quite a catch for me to even contemplate taking things to such an intimate level.
I've never turned down a girlfriend though... I think it's very important to take care of your partner even when you're not feeling it. I know how it sucks to be turned away by your partner when you want to be with them. I try to treat my partner the way I would expect them to treat me.
I'm not sure if this counts, but I was offered a BJ (for cash) by a quite attractive girl on the street. She had tweaker's eyes, so I politely declined. She was thus offended and shouted out so after me.
I probably wouldn't sleep with a virgin on the first date because while I personally don't care many girls view it as a big deal to lose their virginity so I wouldn't want a girl regretting sex if we break up quickly.
Yes, because it keeps girls on their toes to throw them a curve ball... well if you want to really get them hooked on you, you do little things like that to keep them from getting to comfortable too soon.