The following contains major spoilers for Iron Man #20, available now from Marvel.
The Marvel Universe has long been home to some of the most complex, often convoluted relationships in all of pop culture. While there have been plenty of iconic relationships that have endured over the years, even more of them have been doomed to fail from the start. Of every costumed crusader, none is more familiar with that fact than Patsy Walker, aka Hellcat. In fact, Patsy is so well acquainted with marital bliss gone wrong that she turned down the proposal of a lifetime, and that's something Tony Stark is going to have to come to terms with sooner or later.
Since stepping away from all of his powers, Tony Stark has embarked on a new chapter in his life. As seen in Iron Man #20 (by Christopher Cantwell, Angel Unzueta, Frank D'Armata, and VC's Joe Caramagna), his first step was going back to rehab and finishing his treatment, and Patsy Walker picked him up just like she promised. What seems like only moments after reconnecting, Tony takes it upon himself to pop the long-awaited question to the indomitable Hellcat. Unfortunately, after taking some much-needed time to think about her answer, Patsy decides to let Tony down as gently as possible. Of course, this isn't a decision she has come to easily, but rather one that she hopes will protect both herself and Tony from the seemingly inescapable trend of mortifying matrimonies that she has been in.
Patsy Walker has been a part of the Marvel Universe for nearly six decades and, she has struggled with toxic romances for nearly as long. Robert Baxter, Patsy's first husband, went on to become the monstrous Mad-Dog just a few short years after their marriage fell apart. Eventually, Mad-Dog returned to ruin Patsy's second wedding, but he was ultimately defeated. Looking back, things may have played out better for everyone if he hadn't been, especially since Patsy's second husband was none other than the son of Satan himself, Daimon Hellstrom.
The marriage between Patsy and Hellstrom was even more disastrous than her last. At a time when the spawn of Satan's powers were waning, Patsy took it upon herself to summon his father to save him. This demonic pact was successful in saving him, but at the expense of Patsy's soul. After being violently freed from a coma-like state, Patsy's spirit ended up fighting a never-ending battle alongside former fellow Avenger, Mockingbird, deep within the realm of Mephisto. As horrifying as these outcomes might be, it isn't even the most shocking thing that forced Patsy to turn down Tony's proposal. There is a deeper problem that both she and Tony have always struggled with.
Nearly every hero in the Marvel Universe has had to face the often sobering fact that they cannot save everyone, and that trying to more often than not, leads to ruining their own lives. Tony knows this more than most after wielding the Power Cosmic in his pursuit of benevolent godhood. That, and his other, more personal demons, were exactly what drove Tony to make such drastic changes in his own life. It could also be the reason that he proposed to Patsy in the first place, and that's a risk she simply cannot take.
In their world of superpowers, otherworldly threats, and unfathomable stakes, a lack of foresight can cause a fate worse than death. The idea that there is any reason other than true love behind Tony's marriage proposal, or that there could be another reason for her wanting to say "yes" is too much of a risk for Patsy. The last thing either of them needs is a failed marriage, and when considering the relationship they have cultivated already, taking that risk just doesn't make sense. This isn't to say that the two will never tie the knot, but that if or when they do, it will be because nothing else made sense. That is, assuming they don't fall apart before they can get to that point.