Sometimes things are not so cut and dry as what it appears they should be. So her feelings can be very normal in this situation. You don't mention your age, which I'm assuming you both may be young and I'm also assuming no children either. But either way, her behaviour is not what it should be with her being married. It's one thing to be going for a girls night out etc. But kissing another man is wrong.
She may be feeling pressures of life or she may be missing the party life. Either way you can't "force" her to do anything she doesn't want to do or force her into feeling a certain way. If you love her and think you want to be with her still, then give her X amount of time to figure it out. If she still can't figure it out by the deadline, then you need to make that choice so you are not wasting your life away waiting for her. If it means filing for divorce and getting on with your life then that is what you need to do. Or you can sit back and just wait. But in the end, you would be hurting yourself more.
You state you don't understand, as someone either loves someone or they don't. It doesn't have to be an issue of love.. she may just not be feeling complete. And feels as though she is missing out on "fun". And still loves you.
You should sit down and perhaps write her a letter with how you feel and what YOU want out of the marriage. That means if her going out doesn't feel right for you, then say it. But if you continue to let her go out and come when she wants, things won't change and she will continue as she is. And I don't think that is what you want. If you find out she wants something totally different than you, it's very possible you both may have to go your separate ways. If she is really wanting to work thing out. Perhaps you both can see a counselor together to work on a better marriage. But it has to be a two way street as it takes both to make it work. Not just one.
So let her know what is acceptable and do this with out arguing. That is why I suggest writing the letter.. That way you get everything out that you feel needs to be said. Then this gives her a chance to read the letter. And after you give her the letter, give her a few days to think about everything you wrote. And then meet somewhere to talk about it. A quit public place, which can help with nobody fighting. If she can't do as you wish or if you both can't find a middle ground, then it's best to know now, so you will know how to proceed with your life as well.
I hope this helps and gives you some thoughts to ponder. If you need anything else please feel free to ask. If you found this helpful don't forget to click the *ACCEPT* button and please take a moment to leave me *POSITIVE* feedback.