Before giving any real advice, I'd like to know more about the kind of places and/or how you met the guys you described in your opening post. How did the conversation get started? Did you immediately get asked out or was it after having been around them over a period of time etc ?
Depending on the setting (where when how) you first met, certain assumptions may have been made on the part of the guy without you realizing it. It would be good to know that detail so as to have a better picture of what's going on.
Also, if the guy seems a bit too at ease when he approaches you or is rather quick about asking you out after just having met, he may be a bit of a player. Although it's flattering to have someone take an interest in you especially if you're at all nervous or shy and you find it's easier for when they are a bit aggressive about it, If he's just going from one girl to the next and you're looking for something more long term, it would be good to figure that out up front by gathering some clues about the guy and how he operates.
When someone asks you out, you could screen the unwanted types by letting him know you and some friends are going to be at such and such a place on Friday, then saying " Maybe I'll see you there ? " If he's just looking for the here and now, he won't likely be interested. If he does show up and you hit it off and he asks you out again, you could suggest doing something similar. If he's interested in you and not a quick hook up, he'll agree to seeing you any way he can. He'll also be less likely to assume that you're a girl who is more causal about dating than you really are and that he needs to play his cards carefully if he wants you to take him seriously.
Jerks and clods who ask for or send X-rated pics are a dime a dozen and they're the ones who are the most vocal and aggressive. To them you're fair game...you're supposed to know the score. There are plenty of guys out there who know how to behave but they are more reserved and harder to spot.