I feel like I was unfaithful but I wasn't?

Alright, so my best friend kissed me, no I didn't encourage him in any way I've told him before that I'm with my significant other,  and I pushed him away and told my significant other, and they're not mad at me because I didn't initiate it or encourage it, but I can't help but feel horrible that it even occurred? I feel like I was so unfaithful and just a horrible person.
Was I unfaithful? I feel so nauseous even thinking about the whole situation.