You state that everything is going well in your relationship, except this incident. I would ask yourself and your BF if this is absolutely true.
I had a similar incident a few years back. My SO and I have a difficult relationship, it borders on abusive to me, though being male and aspie, I just take it. It was pretty bad at one point, and after a particularly bad night of her berating me, I learned that an old friend of mine was divorced and was asking about me. We had expressed romantic interest in each other, but never acted on it.
Now, I agree that we Aspies are "loyal" to a fault, but I question the true root of that loyalty, knowing how difficult we find building and maintaining relationships.
In my case, I felt that maybe my loyalty was miss placed with my current SO, and that maybe the true loyalty was with this old friend. I did still have feelings for her, even though I had no contact with her for several years. And if she was still interested in me, then maybe questioning my loyalty to my SO was not unreasonable.
I didn't act on this new interest, other than a couple of phone calls, but had it continued, I think things might have gotten more involved. Being an "honest Aspie" I was unable to keep this renewed "friendship" a secret, and all hell broke loose.
Chastened, or chastised, I stayed with my SO (we have significant entanglements) and I broke off all contact with this friend and anyone involved with that time of my life. Things are finally back to "normal" with my SO, ie. she's just mad at me all the time about the little things in life, not some long ago lapse in "fidelity".
My point is, as I wrote early on; are you sure that everything is just fine between you two?