I think this scenario has everything to do with OD. I had a female friend who did OD and it almost drove her to a nervous breakdown, even though she's a lovely sane person- the constant pattern of keenness, the few dates, the getting closer, the sex, then the rejection. She kept saying 'is it me, what is it about me?' when in fact it was absolutely about the men and the OD pattern.
I think there are lots and lots of men out there, some well into their mid-thirties even forties who in the past would have settled down as all their mates were doing so. They now have the opportunity to prolong their dating life and start to see the endless parade of women in their twenties keen to date them (misguidedly thinking these are the men that are more mature and going to want to settle down). In fact, the opposite is true, they can't believe their luck at this opportunity to duck out of all that comes with serious relationship and still get female company and sex.
Hence, after a few dates, and one or two shags (which they have to be extremely keen on the woman as it is hard to win the nice women over online), they then realise that the woman thinks they are now in a relationship and dump. It's not about the sex itself (they may have sex a couple of times before scarpering, or even try to come back later on for a FWB shag/booty call late at night), it's that the sex signals the shift into a more serious relationship, they get cold feet, and scarper, leaving the woman who thought the whole thing was growing and developing nicely utterly bewildered.
My poor friend in the end had to give up OD after several of these repeat rejections, it was battering her ego just too much. She was also quite susceptible for interpreting sex as love, or the start of love, and that meant she was more hurt than someone who is just in it for the fun by them doing a runner after a few sexual encounters.
I think this behaviour was common before OD, but it's now in epidemic proportions, as there's an endless supply of women with whom they can have the chase, the dates, the convo, the sex but don't actually have to have a full relationship or be mature- into their forties, and so throwing women back in the pool and trying for another fish just becomes habitual.
I think lighteninggirl? said all this in a lot more succinct way earlier in the thread, but I just wanted to say it's not you, it is them! (and my friend met a lovely guy and settled down, just not through OD)