Perhaps they are into swinging?
You should have walked over to his date/gf and planted a great big one on her lips.
Bonus points if you two walked out together holding hands and left the them standing there watching you go.
Funny you should mention swinging, since I was introduced to a young lady that same night and when she walked away my (ex) girlfriend stated, “she is a swinger and she came over to check us out.” Then my ex turned to me with her finger pointed and said, “we will never do anything like that!”
This was prior to the kiss I’m talking about. And this is an indication that she does have boundaries.
Her behavior was inordinately inconsiderate.
Should have left her ass at the party.
I have several friends, male & female, who will kiss me this way. At first it was a bit awkward, but now having known them for 20+ years, I know it is just their way–they are not coming on to me, aren’t romantically interested, and don’t want to swing. They would do this in front of my husband–and the women will kiss him on the lips too. Again, it is just their style and it doesn’t bother either one of us a bit.
That said, it clearly does bother you & if that is also your gf’s style (& she isn’t willing to change), you probably aren’t a good match.
Incidentally, my family is like this–they all kiss other family members right on the lips. Took some getting used to for my husband…
Yeah, it doesn’t really matter how we’d feel about it.
You’re clear on how you feel. She’s made it clear how she feels. Your two positions are not compatible, so there you are.
Don’t overthink this. Your relationship has been on-again off-again for reasons, which are reasons enough to move on. You can’t change her. She can’t change you.
Adding to the chorus of opinions that what matters is your lack of compatibility on something important to you. What other random folks consider “the norm” doesn’t really matter.
Best of luck to you in future relationships.
Thank you. I’m sad, but I know this is for the best. 
Welcome to the Straight Dope, new member! It’s always nice to see people jump in, feet first, making their very first post on the board be about a complex sexually titillating relationship problem they’re having. What a way to make a first impression! What a way to get people riled up and talking about them! What a great way to solicit advice for a real problem!
Anyway, if I haven’t already welcomed you, welcome!
I always wonder about these people who would consider it the norm to attend an event with a significant other and proceed to give an extended kiss on the lips to someone of the opposite gender.
I’ve been around, and I have ne’er seen this norm.
She said we, as in the two of you together, would never do anything like that, she never said she would not do anything like that.
I she is kissing other men that way I can’t imagine she is too popular with the other women in the group.
This
Pretty much this. You’re “making too much of this” in the sense that you are still debating what to do. This is a pretty black and white situation.
And your ex sounds like a bitch.
I’d agree with her, in that it is “just her way”. “Her way” of what?
A committed gf would not be doing so much of this. Her response to your discomfort “You’ll have to deal with it.”
You are held in small worth by this woman.
You should have then. You *definitely *should, now!
I’m really not sure what response you want here. Would you like us all to validate your feeling that your ex shouldn’t have acted this way? Well, some here are, so you’re getting that, but I won’t be one of them. She should act as she pleases in her dealing with others, and was quite correct, in my opinion, (especially at the age of 49, when she’s presumably decided what suits her best in her personal behavior) in refusing to alter for you.
On the other hand, you were quite correct in deciding that this didn’t suit you in a gf, and in ending the relationship. At this point in your life, you should have learned that people are who they are, and taking that personally is a losing proposition. Either accept them, or move along to someone you can.
Whoever is right or wrong in this case (which I’ve noted down on my list of “weird shit I may still experience in relationships”), this is probably the bottom line.
Not too much. Not too little. Juuuuust the right amount.
I’m not entirely convinced that people who do these long but “platonic” kisses aren’t getting a thrill out of it somehow.
I am entirely convinced. It’s fucking childish.