Weddingbee boards

This happened last night at a party with my HS friends who are all invited to the wedding. Guy 1 has a serious girlfriend of over 3 years who I know fairly well and see every time I see him - she is invited to the wedding. Last night Guy 2 comes up to me and sort of half jokingly makes a comment about how I invited Guy 1's girlfriend, but I didn't invite his even though he's been dating her longer. I must've look hurt or something, because he immediately apologized and hugged me and said he was joking - he spent the whole rest of the night saying he didn't mean it and to just forget it.

The thing is, he has been with this girl on and off for many years, but I've only met her perhaps twice. They dated on the DL for a long time because she had another boyfriend, or he was seeing someone else too, etc. etc. I never thought they were serious and we were only inviting people with guests if they were

a)married/engaged/very serious

b)living together

c)we knew both of them well

This applies to Guy 1's girl for sure, but to be honest I didn't make a conscious decision not to invite Guy 2's girl - she wasn't even on my radar. I didn't even think of her one time. Now I feel guilty because it must have been bothering him for him to even bring it up and I'm afraid I offended her. She's a real sweet girl. Should I just apologize and invite her? Am I a total d*ck for forgetting her? Ugh. My first real guest list guilt. TIA ladies.

Thanks ladies. I guess I'll call him today and talk it out. Hopefully she's not too upset.