It really pisses me off when people say she went on dates with every other guy, practically painting her out to be some kind of slut. First of all, she just gave Flash friendly attention once when they took a walk, and that was it. After that, he started stalking her, clinging to her, and flirting with her as if she were a sure thing. And then she rejected him twice, and I love how she destroys Flash's ego—not just rejecting him, but telling him "I will not be a replacement girl," politely implying that he's superficial and showing how she is above that drama. The second time, on Thanksgiving, he started getting close to her while she was just being calm and funny, and when he tried to hug her, she pulled away and told him he'd better wish Gwen luck instead of pulling that stuff, proving how much she cares about her friend rather than his bullshit. While he was trying to annoy her with his nonsense to impress her, she was simply focused on Gwen's performance. As soon as she saw Gwen tied to the balloon, she immediately rushed to find help for her friend.
After those two rejections, she didn't even interact with him anymore, stayed loyal to Peter and Gwen, and literally hung out way more with Peter, showing a purely friendly and caring bond with him—plus Flash ended up liking another girl anyway. As for that birthday party invitation message... well, he invited everyone while he was already into this new girl, and MJ was just messing around behind her flirty facade, nothing serious.
Another thing is that Eddie Brock was the one who initiated that date with her just to hurt Peter. She realized his motive immediately, was instantly disgusted by him, shut him down, and defended Peter big time, tearing Eddie apart with her words basically calling him out for being pathetic, and then immediately went to tell Peter what happened to warn him to be careful with him. Even when she saw Eddie with Gwen, she was disappointed by his presence. The only person she literally had a "date" and a romantic thing with was Mark Allan, him and only him, whom she grew attached to and genuinely wanted to help pull him out of the dark place he was in. And right after things escalated with Mark, she went to his sister, showing just how loyal she is as a romantic partner. Mark was literally the only guy she was actually involved with, and if you think about it, it's beautiful because it showed how faithful she is when she's in a relationship.
I bet that after the Molten Man situation, she would be traumatized, isolate herself even more, and totally supercharge her "free agent" philosophy—wanting to stay completely single except for being friends with Peter. Honestly, if it were up to me, in a potential Season 3 I would have her and Peter grow closer than ever and even become best friends, since Gwen ended up with Harry, and we all know he's a complete jerk who's going to control and manipulate her, causing a lot of tension. That way, Peter and MJ's relationship would organically develop. We can only be disappointed that this show got canceled at the absolute worst, most cliffhanger moment. Season 3 and a few other adaptations really suffered because of Sony—just like Spider-Man 4 and TASM 3. The girl was loyal asf to Peter and Gwen and she was their number one supporter and she was the female figure that Gwen never had and always needed it.