I went through this exact thing about a year ago now. Everyone experiences things differently, but here's how it went for me:
I (21F) loved my SO (22M) more than anything. He was my first serious SO, but I genuinely believed we were going to get married. We had been together for three years. We had that fairytale relationship all our friends were jealous of. But then I started getting antsy. I hadn't slept with anyone else before. I hadn't been in any other relationships. I knew it was important to me to find out what else was out there.
We tried an open relationship to start--my SO said he didn't want to know about my "adventures", but he was fine with me doing it. He was at liberty to do the same. I didn't want to have sex with a complete stranger, so I started talking regularly with an old acquaintance before proposing casual sex. Honestly, it was awesome for the first two weeks. I was getting amazing sex with a new guy, i still had my SO, it was perfect. This new guy and I had AMAZING sex, so we took every chance to meet up, which happened often. Then, some days my SO would unwittingly initiate sex after I had been out with my new guy. So I had the guilt of either sleeping with my SO after having just had sex with my new guy, or avoiding sex with my SO. It just wasn't a good situation.. it cause a huge rift between my SO and I. I started wanting more out of my SO and my sex-life, things he wasn't willing to try but my new guy was. His libido was lower than mine, yet my new guy had a high libido as well.
Then theres the emotional side: This new guy and I talked frequently. It started as general get to know you stuff. He was well aware of my situation and that I had no intentions of ever leaving my SO. He was always very doting--he'd ask about my day, talk about silly things, personal things... Everything my SO had gradually slowed down on as we had grown more comfortable together. I loved this new attention. I assured myself it was mild infatuation, and kept things going. I started noticing things I liked about this new guy, that my SO would never do for me. I started realizing that my SO wasn't as perfect as I had thought. That WE weren't as perfect as I'd thought.
Guess what? I ended up falling for this new and exciting guy. Surprise surprise...
I ended up separating from my SO with common intentions of getting back together after we explored the dating world. I don't think we'll ever be able to go back to what we had before.
TL;DR In my experience: -Open relationships will change if not destroy your current relationship. -Casual sex rarely stays casual--one night stands or nothing -I found it imperative to my personal development to get out, sleep around, and date other people. I've grown so much in the short year I've been separated. -Maybe if you're feeling this antsy, there's underlying problems in your relationship that you haven't yet realized are problems.