I think you should talk it out in IC and go from there.
I do think that a key to R is full honesty. Which means open communication.
I do not think it matters if you were engaged, married, or had yet agreed to an exclusive relationship. If you kept this from him at the time and have kept it from him since, it is one of those things you do need to be open about. I think what does matter is that you were in a relationship with your (now) husband and had a secondary relationship going on that you did not disclose to him.
If I were in those shoes, I would have wanted to be told about those kinds of situations - even if we were not engaged or married. It is still a breach of trust.
(sidenote: I never casually dated, my husband and I did not start with casual dating. So the idea of secondary or tertiary relationships may be a complete given in some dating scenarios where it's just assumed people see others until there are spoken words.)
I think introducing it quickly can go one of two ways - either it helps open up that painful full communication and honesty. Past and present. Or it could go that it shifts focus from your husband's actions to your past. That'll depend a lot on him, how he's doing with remorse, and how he processes it and is affected by it.
[This message edited by Lark at 8:06 PM, July 21st (Monday)]