Because I'd say they don't have the same views at all.
Holden Caulfield may be called conservative and considered to have a silly, unhealthy view of sex, in the eyes of somebody in 2013, and his attitude might be attributed to the time he lives in- the 1950s. But what is conservative, silly or unhealthy about it after all? Nothing is. Neither is he disgusted by sex, as I have seen some people write somewhere in the internet. What Holden disapproves of and objects to is not sex itself, but loveless sex, as seen in his disgust with Stradlater, who dates and sleeps with lots of girls without really knowing anything about them.
[If you insist that it is this way of thinking that is conservative, this would be another story. In that case I have to say, it is conservative to insist on sex between husband and wife only and to condemn premarital sex but I do not think it is conservative to prefer lovemaking as an act of love to sex as a physical act, and a person is not necessarily more open-minded and modern and liberal just because they view sex casually and have sex with anybody. I see these 2 views as 2 different ways of thinking rather than a conservative vs liberal opposition].
(Besides, when Holden tries it and a girl says stop, he stops. He always stops. I find that cute).
It's different in the case of Quentin Compson. Maybe Quentin's more likeable and sympathetic. Because he's more tragic- he grows up in a family where the father is alcoholic, the mother is hypochondriac and selfish with no real love for any of her children, a brother is retarded and another brother is selfish, cynical and spiteful, and the only person left, his sister, whom he loves most, strays away from his idealism, his set of values, whereas Holden, in spite of his intelligence and sharp perception, in spite of his loss, is considered by many as annoying and whiny because he keeps saying "phony" and doesn't seem to like many people. And yet, Quentin's the more complex one, the agonised, troubled, neurotic one, the mixed-up one. Sick, even.
Put it this way. Quentin's attitude is much more complicated, because whilst most people are correct in saying that he lives by the Old Southern code of honour and chivalry and thus finds it impossible to come to terms with Caddy's sexual promiscuity, with her being a 'whore', a 'slut', another side to it is his being a virgin, his emasculation. It should be noted that at some point in his chaotic thoughts, Quentin wishes that it is he that is an 'unvirgin', not Caddy. That might mean he's being protective of his sister (willing to suffer for her), but lots of times his protectiveness exceeds the 'normal' line and, though Quentin loves his sister and wants to protect her and doesn't want her to suffer, at the same time, because he gets so obsessed, it appears that he wants her all to himself under his protection and that he projects into her his idealism, his set of values, his ideas about purity and innocence and all of his expectations that when she disappoints, he collapses.
It should be noted too that Quentin talks about incest. It can be interpreted in various ways. He loves his sister so when he thinks she has sinned and will go to hell, his logic is that he should also go to hell to protect her, by committing incest. But I think if he thinks she will go to hell because of her sin, he can also go to hell for the same sin- premarital sex with some other girl, and yet, he talks about incest. Does he desire her? Does he want to possess her? Is his love for Caddy more than love of a brother for a sister? I can't say. But he thinks about incest and talks about incest, I don't think that should be ignored.
PS: Actually the 2nd section isn't particularly easier. More accessible but still challenging. I mean, the 1st section is narrated by a mentally handicapped guy and the 2nd section's by a neurotic, terribly depressed and mixed-up guy, with lots of stream of consciousness. What the hell, Bill. What the hell.