I agree with moms who say it can be perfectly normal and innocent behavior. However, I would make a point to mention it to the teacher, and possibly the parent of the other child involved.
I say this because I work as a teacher, and we monitor this kind of play carefully. I had one child who wanted to plant "movie star" kisses on the girls in the class. Apparently, his father had let him watch some rated R films, and he was trying to get on top of the girls and kiss them passionately... it was inappropriate play, and we would redirect this student when he wanted to play this game.
Similarly, in another school, we had two girls want to play "husband and wife" and that was fine, but they started to go hide in corners and under blankets, and look very embarrassed if a teacher came to check in with them... we had to stop that, too, because they were doing a lot of mouth kissing and touching. Children observe a lot of the life around them, and sometimes they see romantic gestures from adults, and they want to imitate them. This, in itself, is not a problem. But it does need to be monitored.
So, I would mention it to the teacher, and also talk to your daughter and let her know it's great that she can play getting married, but (like you already said) we don't really kiss friends on the mouth at school, because it can spread germs. Kids naturally want to hug and kiss each other, and that is very sweet and innocent, most of the time. In my last school, I would tell the kids that they can kiss on the cheek, and only if the other child said it was okay. No mouth kisses. And, of course, hugs are usually fine, unless too rough or something.
If kids like playing marriage, I also encourage them to play it with dolls, and the dolls can kiss, if they want to. Your daughter will naturally keep the fantasy alive inside of her, but she will also need boundaries, even at this young age. That's just my opinion, from being a mom and a teacher.
Best wishes!!!