When Does an Open Relationship Start?

So, my boyfriend and I are monogamous and have been happily so for the last years. However, we recently came to the arrangement of making out with other guys at parties while the other one is present. I know this is something tame to do, but we had a discussion about it and came to the conclusion that we'd still understand our relationship as monogamous.

Which made me start thinking... when does a monogamous relationship end and an open one actually begin? Now sharing a guy to make out with while drunk - I think most of us can agree, that's not really an open relationship. Having one-on-one sex dates with strangers you found on a hookup date and giving your partner but a heads-up: I think that's the textbook definition of an open relationship. But where's the line?

Obviously this is subjective, so I'm just interested in your opinions. Is it when you do ANYTHING with a third person? Are threesomes an open relationships? What do you say?

To be honest, I'm also just curious where this relationship is heading.