MJ is being written as immature and whiny. It's like she's degenerated as a character from how she was portrayed before. Peter might not have loved Kitty, but at least she was actually helpful and supportive instead of making him worried all the time.
The scenario of "I'm happy with my current girlfriend, but I'll dump her for this other girl who I had a passionate romance with that we mutually ruined for all of eternity with a series of terrible mistakes and decisions" isn't unique or unrealistic, but it's frustrating to see these characters who have supposedly grown play it out. Bendis isn't treating it like we should be happy he kissed MJ before even breaking up with Kitty, but it's not being portrayed like it's unhealthy and I really think it is. I mean, Peter and MJ are in high school, but it's like they've thought since the beginning (seriously, check out how many times they've talked about getting married!) and still secretly think that they're Meant To Be.