Initially I said no that I wasn't prepared to sleep with someone I was dating and we somehow got into this debate about how I need to go with the flow and do what feels right and if I wasn't ready for sex he thought I wasn't ready for anything else. I told him it was about respecting myself and I wouldn't sleep with someone who I am only dating especially someone who's dating others, he said he'd last had sex 3 weeks ago just when we started dating. I felt that I was having to justify why I wouldn't have sex with him and that he was trying to convince me I was wrong. We would drop the subject but he would bring it up again then say I was picking on him or questioning him, when he brought it up again.
This sounds pretty manipulative to me, and I think he sensed that you probably weren't used to enforcing boundaries, so he kept going. I was sexually abused as a child. As a result, I have never felt comfortable saying "no" to sexual activity or even really understood that I was ALLOWED to say no, even when it was totally unwanted. I was programmed at a very young age (like toddlerhood) not to have any type of boundaries about sexual activity or my body. I had it ingrained that I had to go along with whatever guys wanted and it has gotten me into some very bad situations.
Not saying that's your experience, but this seems to be a pattern you've repeated frequently since your axh left for the last time. I used to do something very similar in every single relationship I was ever in. Slept with the guy almost immediately whether I really wanted to or not, and then kind of just hung on for the ride, whether it meant him never calling me again or some kind of ongoing dysfunctional relationship. And I never just casually dated people and didn't sleep with them. I never really knew how to do that. Sex was the only way I knew how to relate to men.
That's one of the biggest reasons I've avoided dating. Even though I'm working through my codependency, I'm still not sure enough of my new boundary setting skills to apply them to a romantic relationship yet.