Ok, here are some of my thoughts:
1) I think age 7 is a pretty common age when kids (girls especially) start viewing the opposite sex as more than just a friend, and they start to realize that, eventually, those opposite-sex relationships will develop into more. I can remember that we started having "boyfriends" in second grade or so. I mean, it was stupid and it never amounted to anything (I don't know anyone who married their second-grade boyfriend).
2) She probably has noticed some girls around her age all ready developing breasts. I was at her age, unfortunately. This is an obvious outward sign that some of her classmates/friends are starting to "grow up" and it can be exciting and intriguing for them. OMG! The bru-ha-ha that ensued when a girl came to school wearing a bra for the first time in elementary school! It was practically a rite of passage.
3) The good thing is that most seven year olds probably have no clue what those "something more" relationships are going to mean (sex, marriage, etc.). Some have a vague idea that there are people who are boyfriend/girlfriend, but there are some kids who are more worldly and can start over-sharing information that is questionable for seven-year-old ears. It might be time to start having "the talk" with her or at least reinforcing things you've all ready talked about before.
4) It's probably not that she's obsessed with marriage so much as what marriage represents: people who are married are grown ups and grown ups can do whatever they want to do. At least, that's what kids and teenagers think.
5) She might not be aware 100% that she's able to wrap these boys around her finger--she probably doesn't even realize that she's being a little flirtatious with them. She just thinks she's being friendly and nice. But she is probably perfectly aware that she is able to get boys to do things for her and that can feel empowering for anyone. If it were my daughter (who is blessedly only three right now), I would encourage her to think carefully and make sure she's not "abusing her power". Remind her that these boys are her friends and she should treat them kindly as friends--she should not ask them to do things that she wouldn't ask her other friends to do. You and I both grew up with girls who learned early on to manipulate boys to get what they wanted and that's not how you want your daughter to be.
It sounds to me like she's being pretty normal and she's just using marriage as a way of expressing herself because this can all be pretty complex for a seven year old. Encourage healthy relationships and friendships. Maybe she's seeking a little more responsibility and freedom to be a little more "grown up" and maybe you can give her that. Maybe it is time to go pick out a training bra? I don't know--that's between you and her.