My girlfriend told me after class...

My girlfriend told me after class, her ride to work was a co-worker and on the way there he asked if she'd like to see his dick. She laughingly responded "Oh yeah, sure." but he then pulled into a parking lot and whipped it out. She said she just leaned away and covered her face, but she could hear him stroke a bit, and she didn't think he'd actually do it and froze because she was afraid he might try to get her to do something if she so much as looked at it.

I've been completely pissed off since, but she did tell me about it within seconds of it happening. I don't want to think she's hiding anything, but this shit flashed me right back to fucking middle school. I remember cutting contact with a girl I'd been making out with when she told me she and her friends were hanging out, and one of them yanked out his dick and they were all "really hyper" so she just "gripped it and held it a bit" I was so pissed at the time I blocked her on everything and never spoke to her again. Years later I dated a girl for two years who cheated on me for two weeks with some dipshit from a chatroom and my first public acknowledgement of this was seeing this stupid post on her facebook where she'd set up a date for "international touch a penis day" attended solely by her and him.

What's a valid reaction to this? She told me right away, I want to believe nothing happened, but I also want to fucking deck the guy.

She quasi-cucked you to feel out what your reaction would be like if she cucked you for real. She will probably try to do so very soon

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This.
You need find him, and ask to see his dick.

This
Kick his ass and show him why he shouldn't fuck with you.

It's a bit gay but I'm pretty sure that you could probably get her to complain about harassment, unless she is actually just straight up fucking him. Which is the most likely scenario, let's be honest.

Firstly, make it clear to your girlfriend that if such a thing happens again, your relationship is over. Explain that you are not a self-hating loser and that your relationship is predicated on mutual trust and love, and that it is not negotiable.

Secondly, as everyone else is saying, kick the shit of the man who is clearly making moves towards your woman. Do not confront, do not talk, beat the shit out of him until he is on the ground and submits to you.

If you aren't the physical type, then, well, you have a problem. It is important, though – as you note – that your girlfriend told you about it. Deal with the hand you've been given, user. Don't be an idle little bitch, whatever you do.

If you need to "explain" this then your problem is worse than you thought

Well, he should put it in plain words that he isn't going to allow these things to happen. The more simply and bluntly he words it, the better. Communication with women is key; they must know the rules. One of the main factors in failed relationships where the woman cheats is because the man was not strong worded or willed enough, and the woman instinctually – remember, women are creatures of impulse – interpreted that as weakness.

I'm a bit older than her, so the relationship hadn't really been publicized, and she's not really close enough to her co-workers that they discuss relationship stuff, so he most likely didn't know about me, but I told her two things. First, I told her about the girl I completely cut contact with in middle school. She said it was disgusting the girl did that and expected me to see it as no big deal.

Second, I made it clear I was pissed with the guy and just asked her to let anyone who flirts with her know she's taken.

I'd still like to deck the guy, but I'd kind of have to interrogate her a bit to get his name, and I don't want her to feel like shit if she really was totally innocent here.

I was thinking I might just show up at her work -which she has invited me to do numerous times- and kiss her in front of her co-workers. Not so explicit she'd get in any trouble, but enough of a show that it was clear she was totally spoken for.