Is monogamy really for me, or am I overreacting?

I've been in a relationship for about 8 months. It's been really great, and I'm in love with the guy. He's a non-monogamous person, which I knew from the beginning of our involvement. However, recently it has started to get to me—and the reason for this 'delay' is that I think, in the first months, he was hiding things from me (his dates, hook-ups, whatever). When he started to tell me (about six months into our relationship), I felt cheated on—even though I realize that's not the case at all. I also felt lied to, as if what I was living and feeling during that time was a slightly manipulated version of reality, one in which things were different between us.

To be clear, I don’t mean to impose exclusivity on our relationship—that was never the initial arrangement. It's just that the way he went about it felt disingenuous. I remember the first time he told me: it was a hook-up story that had actually happened during our relationship, but he started presenting it as if it had happened before, and I only realized it was recent when I asked his age at the time of the event.

After that, he began telling me everything, but it became too much for me. It's not about jealousy—I actually believe it's healthier for the relationship if we're open about these things. However, I just don’t want to know all the details. It’s not personal; I’m just usually not interested in that type of story (for example, I rarely get invested even in my friends’ relationships—especially if I don’t like their partner, I’d rather not know anything about them, but I believe this is crucial for him.

The last thing that hurt me was something that has become recurrent: I wanted to spend time with him when we were both free, but he had other plans for that time—he was going on dates. I think what truly upsets me is that he’s such a slut, and I could never (and haven’t been able to) keep up so I feel used. I haven’t been with anybody since we started "dating".

Is monogamy really for me, or am I overreacting?
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