I cheated on a guy I was with for eight months when I was about 25, we had gotten married too (was years ago, before some now EU States were fully in the EU and his residence status was vulnerable, he was supposed to be working a limited number of hours while being a student but wasn't, was working full time etc) which made it more significant and regrettable.
There was no sex but sexual but sexual contact. I had a crisis of conscience and tried to end the relationship around the time but he got v upset, said he couldn't cope etc and I continued it, only to end it to honour with an older man a few months later.
I've apologised to him twice, he's nice enough to have remained in civil contact since then .. but it doesn't really change it being shit.
I also cheated on my now DH in the first few weeks of the relationship (again sexual.contact but no sex) and an emotional affair (one sided emotional affair really and some sexting) before I broke up with him a couple of years into the relationship. We got back together, he copes incredibly well (or appears to) with the knowledge, I couldn't if it were reversed. We are more like house mates now though, and every now and then during stressful periods, he suggests breaking up & divorcing.
The person who said it's cowardly is correct.