FitzSimmons was never a case of unrequited love, even from the start. This wasn't an unbalanced romance, of one pining for the other who was unaware or dismissive. They were mutually interested, but they were both inexperienced, so it was easy to place the work of fighting evil with S.H.I.E.L.D. first. But they were always equals. Even within their differing fields of expertise, they each acknowledged the other was the best at what they do, and they supported each other's ingeniousness- even pushing each other to reach new realms of brilliance. They were - and are - a team of the two best in their fields who inspire each other to be even better, especially better people.
They have certainly suffered together. Their worldviews were shattered by HYDRA's subjugation of S.H.I.E.L.D. They were left to die at the bottom of the ocean by Grant Ward, with Simmons finding strength she never knew she had (something she would continually discover) to save Fitz. They've faced Inhumans, monsters both human and alien, and they've repeatedly been torn apart. Who can blame them for believing for so long that they've been cursed to never be together? Simmons was literally ripped from the Earth and banished to an alien world, where she fought for her very survival. For his part, Fitz never once considered abandoning Simmons or accepted that finding her and rescuing her was hopeless. Fitz spearheaded the all-out effort to cross the vastness of space itself to find Simmons and bring her home.
In the virtual reality of the Framework, which explored the darkest recesses of Fitz manifesting as one of the leaders of HYDRA, Simmons in turn simply would not accept this monster was the reality of the man she loved - and she was right. Even when the evil Fitz was romantically involved with Aida, Simmons dismissed it and saw the real Fitz throughout. Immediately after they destroyed the Framework, they were separated again - by time. With Simmons trapped with their friends 90 years in an apocalyptic future, Fitz rallied and found his best self. He dedicated every waking moment and all of his considerable brainpower to figuring out how to find Jemma. Fitz literally took "the long way" to Simmons, cryogenically freezing himself for decades before donning a guise as an intergalactic marauder to save her from slavery by the Kree. When they fought their way to safety, they fought together as a unit, and they saved their powerful superhero friend Quake in the process.
Does it matter that when Fitz first proposed, Simmons didn't hear her because Kasius had robbed her of her hearing, so that it's Simmons' proposal that is the 'official' proposal? Well, maybe it does a little to Fitz, but in the end, he doesn't care. Because he recognizes the truly important thing: Jemma Simmons is the one for him, and she feels the same. They always felt the same about each other. Now, they're matured, as Agents of S.H.I.E.L.D., but more importantly, as two people who repeatedly overcome insurmountable odds, and they're ready to commit to each other forever.
If Jemma Simmons and Leo Fitz are 'cursed', it turns out everyone should be so lucky to be as cursed as they are. Fitz and Simmons are, in a very real way, the beating heart and soul, not just of Agents of S.H.I.E.L.D. but of the entire MCU. They are goals for everyone to emulate, and despite all that the writers have done to test their romance, FitzSimmons endures as the shining example of perfect love in the MCU. Their road together has never been easy, and no doubt, they'll endure even more trials that will test them, up to and including their wedding day and beyond. But in a superhero universe known for the flaws and failings of its characters, FitzSimmons is a love affair that is continually inspirational and aspirational.
When FitzSimmons' wedding day finally comes (we hope a tragedy isn't what has been teased will 'destroy fans'), there will be pints and there will be gin, and there will be a joyous celebration from their fans that, against all odds, those two crazy, brilliant kids made it.
Marvel's Agents of S.H.I.E.L.D. airs Fridays @ 9pm on ABC.