The question I would ask the Op too, is whether he is looking for a long term relationship where he doesn't have sex for a good while, or if he is looking for sex now.
If he wants a quick, fun relationship with sex now, then I can't understand his attitude towards these women who have had more sexual partners....it doesn't just seem to be worry about his own inexperience, but being put off by the fact they have had casual sex....which is what he now wants to do himself.
If he is looking for another serious long term relationship, where sex isn't involved for a good while, then I can see that he might just think that someone it a significantly different sexual past to himself, simply has different sexual values to himself and like any other differences, just wants someone more similar to himself.
He himself says, that generally he tries to avoid talking about past sexual partners - good move, I would say. Sounds like he unwittingly found out about one of his dates on facebook and the info made him uncomfortable. It's a good reasons why it's best not to know, or not to know until further into a relationship. If the woman was only interested in sex, this would have become clear early on in their dating and if he didn't want that, he could get out then. However, if she was interested in a longer term relationship, and was willing to wait a while like him, their relationship could have become serious and the past not mattered at all.
It's interesting how many people on this thread say their DH or partner doesn't know their sexual past and they don't know their partners, nor care. This seems a good position to take......and will work for most people, but perhaps not for those who want to wait a long while until having sex, or even until they are married.....because discussions about waiting for sex and sexual past is bound to come up then, although lots of detail might still not be needed or helpful. Most people are not in this situation - they have slept with some people and in a new relationship will probably have sex fairly quickly. The relationship will or will not progress due to a number of factors, one of which might be CURRENT sexual behaviour (ie the other person wants to have sex with lots of people, the relationship might break down) but is unlikely to be due to past sexual behaviour, because for most people it doesn't matter.