My (26f) boyfriend (30m) kisses a female coworker (26f) on the forehead goodbye

Throwaway account for obvious reasons. I apologize in advance if the formatting is wonky, I'm on mobile.
So just a bit of context: My boyfriend and I have been together for the greater part of four years, and recently moved to a new city together. We both work in the service industry but have opposite schedules so we don't see much of each other, with the exception of the wee hours of the morning. On top of that, he is extra busy as his place is just opening after being closed for renovations, so his workload has been considerably more intense. All of his work friends are super cool, and they go out for drinks at the end of the night so I meet up with them if I'm able to. He will also visit me at my work if he has the chance.
So before I get into why this bothers me so much, I want to preface that I really like this female coworker. She's very sweet and we hit it off immediately (coincidentally we are the same age and share the same name). I don't feel threatened by her, nor do I care if they hang out when I'm not there. She is not the issue.
However, the last two times we all hung out, as we were exchanging goodbyes my boyfriend kissed her on the forehead/top of her head. I was taken aback the first time but shrugged it off because it was clearly harmless. But it happened again last night, and although I know it means nothing, it upset me. Because of how little we see each other, those little acts of affection go a long way for me, so it hurts to see him do something I consider special and intimate to a woman we've known for a little over a month. I was visibly sad this morning and he kept asking me what was wrong, but I didn't know how to bring it up without sounding irrational and jealous, or risk starting a stupid argument, so I just said I was going to miss him while he was at work, which is not untrue. I miss him terribly.
I feel so silly even posting this, I know how ridiculous this all sounds and I think under different circumstances I wouldn't care so much, but **** man, I've been really patient and supportive of him and his job, I'm really not asking for much. I could use some advice on how to explain this to him calmly, and in such a way that he understands that it's pushing a boundary for me.
Tl;Dr - boyfriend keeps kissing female coworker on forehead goodbye, how do I calmly tell him it bugs me?
Edit: added some words
EDIT: So wow, did not expect to get this much attention. I appreciate everyone taking the time to read and contribute advice/experiences, and I'm sorry I couldn't respond to most of you as I was at work when this took off. But damn, some of you are jumping to some major conclusions. There are a few things I would like to clear up:

1. These kisses are no more serious than pecks, followed with a "muah" sound of some sort. By no means are these drawn out romantic sexy kisses. They made me uncomfortable and my feelings were very hurt, but I never suspected infidelity.
2. Like a few of you have mentioned, the service industry is infamous for sexual humor, which is why I seem to be unphased by this. I still don't condone what he did, but I've seen and heard waaaaaay worse.
3. Had I believed this was normal or ok behavior, I wouldn't have posted this asking for advice, now would I? Put down your pitchforks and be nice, for ****'s sake. I'm not naive. Like I said in my post, we just moved to a new city and I haven't made many friends yet but wanted to vent and gain some clarity, so thank you Dating.mobi. Love/hate relationship, amiright.
4. Yes, he has kissed his guy friends, usually on the cheek, or he'll play a game that some would refer to as "gay chicken"... if that's not self explanatory enough, it's basically two men going in for a kiss until one of them freaks and backs out.
5. He is not an abusive man, nor is he (intentionally) disrespectful, but he does some really stupid **** sometimes, just like everyone else.
6. I know plenty of adult friends that kiss their friends in greeting and departing, but they're single. That's not unusual to me. Boyfriend crossed the line because he is OBVIOISLY not single, and that is where my boundaries come in.

I know I sound incredibly defensive, but some of the responses I read were just downright nasty and targeting my character, which was uncalled for.
A very special thanks to /u/conamo

SOOOOO tired of these Reddit threads. Wish they would all go away.

Tell him to stop...good grief.