Hi there. I just started dating this guy. We knew of each other for a little while, but recently have been talking/texting every day for almost a month. We have hung out one-on-one before, and I can tell that he really likes me. He says sweet things like that and is affectionate.
The problem is that I have these bad memories of dating guys or being interested in guys who used me, kept me around just to hook up with while being dishonest, and now I associate having sex with “the beginning of the end.”
To me, it symbolizes the point where things go downhill, because the guy gets too comfortable. I really like this guy, and most of me doesn’t think that he’ll “switch up” after having sex. The other part, albeit small, is getting a bad case of deja vu, and I don’t want things to change because I’m starting to really like him. How do I stop these reservations based on experiences that happened way before him?
LemonVibe 
For that to happen on such a consistent basis I would have to assume that the sex isn’t good and they may be bouncing for that reason.
VAGINALSEXgODIAM agrees, this could be a good theory. @realrapisback on Instagram. Bye.
The guys only want sex from you? I was that type of guy. It’s how the game goes sometimes…
Discuss your issues with him. If he’s a good man he will be interested in your concerns and not leave. You also may not be dating these guys long enough. Maybe take a few weeks and more than 3 dates before sexy time. Not saying you can’t make out and pet, but slow down. Humans can live up to 115 years in this day and age. Waiting won’t kill him or you. If he leaves because of the wait then sucks to be him.
To add to what 30’s Dater said above … yes, it’s possible the sex is bad, but first-time sex can often be awkward. If you’re dating a guy who’s ready to bounce just because the first time you have sex isn’t great, then he’s not going to be a good fit for you, anyway.
The underlying issue here isn’t so much that your relationships go downhill after sex. The issue is that you seem to be trying to get into relationships with guys who are only looking for sex. So, maybe what you should be working on is how to become a better judge of the guys you choose to date and have sex with.
For that, I don’t think anyone on the internet will have the answer for you. To find the answer, I think you have to take a hard look at what interests you about certain guys. Do you have a specific “type” you always fall for? Is there a pattern to where and how you’re meeting these guys?
I think that would be a good way to start, as far as figuring out why you have this pattern …