IMO it depends on what you want with that girl. It is of course a gray area, but let's divide the interaction into 3 broad categories:
1. Long term BF, provider, potential marriage
2. Short term, causal, pump and dump then disappear forever
3. Fuck buddy, friend with benefits, but still having sex many times
You could probably be a Lover (as described on GC) in all 3 categories, but you don't necessarily have to be
So depending on what you want, you set your frame, and you stick to it. For example:
1. Long term BF: You want to share more, you want to know about her life, you want to know lots of details about her. You want to be carrying about her, sex is of course important but you have to spent lots of time on creating good and functioning relationship. You do something and she does something (you do say 30-40 and she 60-70, forget all about the equality, make her do more than you do otherwise you'll suffer). You don't want to be needy, insecure, immature... You want to be more leading, not only in sex but in the whole relationship. You of course provide, you take care of finances. Make sure she contributes, financially and emotionally, make sure she invests lots of time and effort into that relationship. You want to get to known her friends and her family, be good with them. You want to be assertive, motivated, goal oriented, you should have some passion in your life. You want to be more emotionally stable, manage well your emotions. You want to take her out a lot, different places, do different activities, don't just sit home and play games. You want to keep higher value all the time (such as good job, good income, good friends/social circle, enthusiasm, ...), basically you want to be a good catch. Sounds like lots of effort? It is, but hey, it is you who wants to have great and long term marriage, so you better do something for it...
2. Short term, casual, pump and dump. Who really cares about all of the above? Your purpose is fun, sex. You want to be sexy, have perhaps more of an asshole attitude. Your objection is to have sex only, you don't have to know anything about her in order to get laid. You can be insecure and needy - as long as you don't show it for that night. You don't have to share anything, tell her nothing about you. You don't provide anything. Ok, sometimes it may cost you a dinner or a drink, you have to have place to stay, but that is pretty much it. You don't care if she has children, how is her family doing, who are her friends. Who cares? You lead her, but only to the bed. You don't really have to be emotionally stable, just don't show any drama and shut up about your insecurities that evening, just provide a great time. You don't have to have many friends to get laid, and you don't have to have any social circle either. You don't want to be a good catch, you want to show her that you definitely are not dating material, you are there to get laid only. You could be a total loser, as long as you can get laid who really cares...
3. Fuck budy, more of a friend. Well, it is exactly what it is. Maybe you have some friends in common, maybe you know her for long period of time, maybe you know her family. She knows you, she knows your insecurities, she knows your weaknesses - but as long as you have sex with her, who really cares? You tell her only what you want to tell her, and you may not expect her to tell you everything about herself. You don't want to marry her and she doesn't want you either, except for some sex here and there...
As a general guidance that you can use in all categories, you want to have good value (e.g. good job, good finances, good physical shape, great fundamentals,...). You want to be more emotionally stable, more independent, less needy and less insecure. You want to be more assertive, have some goals in your life, have friends, good social circle... You want to work on your life. The thing is, you don't do it because you want to get her in the bed and keep her as a GF - you do it for yourself, for your personal life. You do it because of you, not because of her...
Back to your question though, say you just met a girl and you do the short term, the casual sexual encounter (#2 above). You hide everything about yourself, for that one night you look like a dominant and confident sexy lover who can walk away from her anytime he wants. You swept her off of her feet, she is all excited, she finally met a man of her dreams. Good for you. But then you change your mind, now you want long term GF. So there is the second day, and the third one, perhaps two weeks or two months pass by... Now you can't pretend anymore, the "real" you comes out. Now she sees all the insecurities in you, now you spill all your beans about you, now you share things she doesn't want to hear about, now she sees all the neediness and clinginess, now she sees that you can't really make any decisions on your own, now she knows that you really don't have any life, she can't even breath because you are constantly around her.... She can't go to the bathroom without you texting her "Where are you? Miss you so much, love of my life"...
For God's sake, really?? NO self respect at all???
As far as the relationship, what you think (talking in general about any guy) and feel about her doesn't really matter, it is not really important - but what is she suppose to think? You are totally different person than when she met you. She sees that everything was just a lie, she sees that you were pretending, pretending just to get laid... You are not really a lover, you are a fake and she is quite disappointed. Perhaps even pissed, more at herself than at you because she was fooled, she has to fall into autorejection, you didn't give her any other choice... And now, as if that wasn't enough, you want to chase her? That just won't work...
So be very careful what kind of frame you set at the beginning, unless you know what you are doing it may come back and hurt you...
