Odee is so right - he is over-reacting.

I interpret your mention of the need for parental consent to mean that you are "brutally young".

Do not get married - you are too young and the boyfriend is obviously too immature. He needs to go through some maturation.

Your grave mistake was your attitude - you "confessed", "felt guilty", "felt ashamed", "regretted", etc. A whole lot of negative self-talk - way too much. Why so - in the opening line of OP, you wrote:

"In grad school I was a part of a friends’ group. I was in a serious relationship with one of the guys of the group. Everybody in the group knew about it."

So you were in a serious relationship that was socially accepted - all the members of the group knew about it. So what was shameful in that?

Plus, I simply fail to understand why everybody knew about it but your current bf did not. How did you manage to conceal it selectively - just from your current bf but not others? It seems impossible. Maybe I fail to read the OP properly.

Also, thanks to Jeffro - I was wrong and you aren't that young. You are referring to the grad school IN THE PAST TENSE. So you are - in late 20s? Not even mid-20s, but late 20s, most likely. And you are one year older than the boyfriend. So does the boyfriend expect a woman in her late 20s to be a virgin? Or did you go to grad school right after undergrad, which could place you into the mid-20s age bracket, as Jeffro guessed. Still - it is unusual to expect a woman who is in her mid-20s and who has already obtained a graduate degree to be a virgin. So something is odd. Maybe you can clarify.

Also, when he told you about his fling, but did not you tell him right away? You seem to believe that relationships should be symmetrical - if he is open, so should you be. Ok, so - being open with him was your idea of doing things right. So what prevented you from telling him the whole truth right away, after he told you about his fling? Maybe I am not getting something, but it appears weird - he is telling you about a FLING. While I personally have no qualms with flings or anything done consensually, the status of a FLING in this society is much LOWER than the status of a LONG-TERM SERIOUS RELATIONSHIP KNOWN TO YOUR SOCIAL CIRCLE. Therefore, if it was OK for him to have had a FLING in his past, it was even MORE OK for you to have had a serious relationship in your past.

Weird! Please clarify.