That you don't want to shackle eachother in the relationship is a good thing, but it wouldn't hurt to have ideals that you both adhere to. These aren't rules more than they are ideals, and it's important that you both understand and appriciate them as individuals and then agree on them as a couple, otherwise you're just imposing them on eachother.
If you had more of this type of communication and agreeability, perhaps you wouldn't be as bothered by this interaction with this other guy.
You are in a jealous situation, so I'd talk to her about your feelings as soon as possible. If she's as loyal as you believe she is, then it's just a matter of letting her know that you'd appriciate it if she didn't reciprocate his feelings, even in jest, as that may lead to him interepreting them as sincere (which isn't entirely his fault if she's entertaining it).
By explaining it to her that way, it shows her you're not acting purely out of jealously, but rather logic as well and a geniuine concern for maintaining the relationship. You're explaining that while it is fine for her to have a unique relationship with him that you don't have with her, there is a level where she needs to know the limit.
She probably already knows this limit, but you letting her know is a validation to her that you care what she's doing with other guys, which shows you're interested in her safety and well being.
Indeed, having some ideals in the relationship wouldn't hurt. You say you have no rules and that rules are pointless, but thats a matter of whether or not they're serving your purpose or not. Couples whose rules are working out for them are obviously not going to complain about them, for example, because they are mutually beneficial.
What you need to do when you have these talks with her, is try and find ideals and considerations (not rules) that are mutually beneficial to both of you. By doing so, you can both feel free to associate with whomever you want while also yearning for the intiative to remain considerate to the other and the relationship in situations that would call for it without feeling entirely obligated to. Because it is a free will issue, theres nothing to resent and you can both be happy with these arrangements while seeking other friendships as well. Relationships are about checks and balances as well, and I'd mind those a bit more.
Can you do that for me? Good.
*faces cameras*
On the next Dr. Damien: dealing with your cross-dressing children whom you suspect were fathered by a space alien.
See ya next time!
*audiance claps as the show goes off the air*
