Given that the OP is from Wales, I'm going to chime in with cultural differences.
The US posters (with the exception of toknow) have been talking about the need for a discussion on exclusivity. After all the time I've spent on various relationship boards, I've come to understand that in the US - one must not assume exclusivity. In your culture, it's OK to see multiple people until it's been discussed.
However, in other countries, such as the UK and Australia - this hasn't been the norm until recent years. Not that long ago, our cultures DID assume exclusivity if we were seeing each other. I could date a guy for two weeks only and there's no way that either of us would date or sleep with anyone else during that time. If we did, then under our cultural norms, we'd be cheating.
From my younger friends, I understand that the UK and Australia are following in the path of the US and people are now having exclusivity discussions and seeing multiple people during dating. However, as with any time of change, there will be people who haven't yet switched over to the new dating rules.
May I add, for all I know, many people in Wales may still be operating under the rule of assumed exclusivity.....and if this is the case, the OP is very right to be upset.
Just hoping for a bit more cultural sensitivity before telling someone that they've got it all wrong :-)
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.