As a cheater myself, and one with fantasies of screwing other men while my husband watched (and actually did it) and who justified flirting with other men, getting free drinks from other men, etc….I think the above response is the best one you’ve received yet in this thread and you should follow this advice.Welcome to TAM @BurntEnds I think based on your OP you know deep down where this is heading. I agree with what @Diana7 said in post #3 that this should not be going on and that the kiss I agree with her has already happened and she was just feeling you out. I hope that is all that has happened, but I am afraid that maybe more has happened. I hope I am wrong here is what I think based on what you have said she told you. She has progressed to dancing and drinks with a guy and then they sit together and kiss as she said open mouth and he gets a little handsy and she really likes it but she stops it before it goes any further. That's why she is coming home and jumping your bones, she is as others said feeling you out to how you feel about things and she is considering going further.
If she starts going out more often and not wanting to jump your bones when she comes in then you really need to worry. A couple of questions then a suggestion for you. These girlfriends she goes out with are they married? Is one a designated driver? Has the frequency of these GNO becoming more often or staying out longer. Do you all share each others cell phones or if she has started keeping her phone with her more using it more that could already be a red flag. Now for a few suggestions one if the phone thing is as I said above I would suggest checking it or your bill to see if their are any numbers you do not know. Next, if where they go is a large and crowed bar/club hire a sitter and after about 30 minutes to an hour go into to the club lay low and observe for yourself if spotted you could always say you were fulfilling a little of your own fantasy. I personally don't recommend the following to the club(just a suggestion).
Finally, here is what I would actually do if I was in your shoes. I would sit her down after kids are asleep and say after your discussion about feelings of real/fantasy you think this is not good. First I would ask her point blank if she has already kissed another guy (on the lips) more than once or more than one guy? I would think after being together for 15+ years you would know if she is telling you the truth. Then I would say that giving this more thought (even if she says she has not kissed or done anything with anyone and you believe her) this is not a good thing for your marriage and your family. That it was your understanding that on these GNO she was just going out with her girlfriends and having some drinks, them dancing together, and doing some karaoke. It now seems to be progressing to areas you are not comfortable with for you or your marriage. I would use the example of how she came home two weeks ago and say what if you get a more drunk than then and a good looking guy comes onto you hard. I would say that you are a guy and a guy in a bar/club who you let buy you drinks and dance with and even kiss is going to want and expect more than that. I would also say is the reason you ask me about how I felt about how far flirting should be allowed to go and what I thought was it because you a) want to do things with other men b) already have done some things with guys and due to you feeling guilty wanted to see how I felt about things or wanted to try and tell me a little of what has happened to see how I would react. You seriously need to sit her down before she plans another one and get this out or I am afraid you will be on here in a few months telling us she had a ONS with a guy from a bar she had to much to drink and was flirting and things just went to far and you will be crushed so you need to put a stop to this now. Let her know how you feel and if she really loves and cares about your marriage and family she will understand and agree. If she does not agree and tries to gaslight you and say you are being controlling and jealous then she probably has already been involved and wants to continue. I would as kind of a compromise suggest that you can hire a sitter and go to a club just you two and share it together. Best of luck! Please don't be a one post and done let us know how things are going. It really sounds like you have a good marriage and family, but I really feel you need to step up here be firm and tell her how you feel and get this stopped before it goes to far, because trust me it will continue to progress if you allow it.