I get messages all the time from guys wanting to talk about sex. I don't think talking about sex is gross and I'm totally cool with having an intelligent conversation about it, but most guys tend to be disrespectful and are obviously asking questions/telling me things because they're interested in getting off, and I'm not into doing that with random people on the Internet.
boys (especially teens) has higher curiosity about sex actually even for girls but girls not showing it often it well be some kind a awkward or like if this girl here always ask and talks about sex a boy here could think that she's slut or something just boys&girls are totally different when it comes to sex topics
those guys see a girl they think is pretty then they messege her and think oh she would totally be up for "cyber" or whatever the hell they call it and then try to get off because they are lonley and pathetic. basically its because they have a sex drive they either can't control or satisfy so they turn to a random girl on the internet and think she will be cool about it.
there is a few ways you can go about this too. you can ignore him, block him, scare the hell out of him in some way or another, yell at him and tell him how pathetic it is that he wants to sex chat with you, say you are really fifty and that is your grand daughters picture and how could you think of my little -insert name here- like that she is too innocent for that.