As soon as a guy gets sexual i go off him?

Now don't get me wrong, I'm not asexual or anything like that. When I meet a guy I like and he acts sweet and romantic and lovey-dovey and all that, I love it. But as soon as he starts "flirting" or hints at anything sexual or suggestive in that way, I tend to go off him, and think he's not a nice person or that he doesn't care about me. Is it because I have been conditioned by society to think of sex as "bad", or "perverse", and that those who want it are gross or perverted? Because I know it's NOT bad, and I get horny sometimes, and I know people who want sex with someone they like are not necessarily bad people. But I just start wanting to escape as soon as a guy starts going sexual. And I feel like I'm doing something wrong, or that I shouldn't be involved with them, EVEN if I would really like to have sex with them. I guess another part of the reason is that because of my, for lack of a better word, "niche" attractions, all the guys I have spoken to this way are online. I think part of this is because of paranoia that he will want me to send him nudes or strip for him or something like that, which I think is dangerous. Even if he doesn't tell me to do that, I feel like that is what is expected, which makes me not want to associate with them anymore. Is this normal and why is this?