Welcome aboard!
You've received some good replies, but I'll throw in my $0.02 anyway.
What he heard was that I was not a pro-gun advocate; what I was really saying is that I am just unsure of my stance on the topic.
We are in defense mode right now. I'm not excusing his attitude, but suggesting that you try to understand that the gun rights crowd has been under attack for a very long time, and severely so for the last 5 months. Naturally, he wants his better half to be on the same side, so it was probably very frustrating to him that you were/are ambiguous. He
should take this golden opportunity to provide you with thoughtful, articulate argument from his side to counter the constant mass media bombardment of sensationalism and misinformation so that you may make an informed decision, but we all suffer from the human condition (especially at 22), and sometimes our emotions get the best of us.
As I see it, you having no interest in firearms should not be a deal-breaker in a relationship. It's always nice for us shooters when our SO partakes, but for many, that is not the case, and we're fine with it so long as our views on guns aren't diametrically opposed. That said, I can appreciate his concern; for many of us, constitutional rights, and especially the second amendment, are paramount. I've dated non-
shooters, but would
NEVER be with an anti-gun person. It just wouldn't, couldn't work. If this subject had not been discussed until now, your fiancée is probably very concerned that you will oppose him on this (to some of us) very salient issue.
I'd say just talk to him, remind him to be calm, and stress that you're not
anti-gun. If you love him, just make him understand that his interest in firearms may not be shared by you, but will not be an issue between you on your end. He should be able to accept that, and you two can move forward with building your life together.