Is it fair to break up with someone in a long term relationship if you found out they cheated YEARS ago?

Anonymous
I've been with my (now-ex) boyfriend for about 5 years, we've been living together for 2, been together since just after high school.

Early on in our relationship, I had a drunken one-night stand with a guy, it wasn't my fault, I was too drunk to even know.

Fast forward to a few weeks ago, we've been talking about getting engaged and I told my boyfriend I wanted no secrets between us, so decided to come clean about it, and well, he broke up with me on the spot.

I'm really hurt, we've been together so long, it was before we were really serious, and I feel like he's throwing so much away from one little mistake that happened YEARS ago, and hasn't affected our relationship in any way.

He's moved out, won't take my calls, I don't even know where he's staying as his friends won't tell me.

Surely after this time together, it's such a minor incident?
Yes, he should break up with you
No, he shouldn't break up with you
Other - Please explain
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Is it fair to break up with someone in a long term relationship if you found out they cheated YEARS ago?
42 Opinion