A., I know how you feel! My daughter was proposed to in Kindergarten! That boy DUMPED her for her best friend!
I was mortified that all of this is going on, but it did leave the door open for some really good talking opportunities.
We were able to discuss what a good husband her daddy is and why I chose him to be her father. She promised me she would not marry someone who doesn't have the same kind of love and respect as her daddy has for me. We talked about loyalty between men and women and between friends. She decided he wasn't husband material. She wasn't upset about the dumping, either. She said she was "happy and sad" because she was happy for them but sad because she wanted to marry him (oooh how I wanted to call his mother!) and we got to talk about how you should never love a boy until he worships the ground you walk on.
All in all, the whole thing existed more between us than between my baby and that 5 year old cassanova. She's back to playing princess and he doesn't go to her school anymore.
I thought we'd be immune to this sort of thing since she goes to a small private school and we have no television in our home. That isn't the case. I think the best you can do is be honest, try to steer it into some critical thinking and self-esteem conversations. You can't stop this thing from happening but you can let her know that she is too young to focus on this. A lot of it is just figuring out what the words mean and using them and building concepts and vocabulary. Some of it is immitating very adult things they see around them.
Whatever happens in parenting, I think, happens at the right time. I say that because it happens and we have no control over what other little boys will say, so we have to accept it as being "the right time" and adjust our parenting to rise to the occasion.
Good luck.