On the other hand, Steve might be more understanding than expected. He knows about Bucky's past traumas and could suspect that something else is at play. So, instead of immediately resisting, he might play along for a while, waiting for the right moment to figure out what's really going on and how to help Bucky.
He could also be very angry. If he feels blindsided by the break - up, he might lash out at Catherine. He could accuse her of leading him on or not being honest about her feelings all along, and this anger could be his way of dealing with the pain of the break - up.
He might be defensive. He could try to explain that he's not homophobic but just comes from a different time with different values. For example, he might say that he's still learning and trying to adapt to the modern world's more inclusive views.
The Avengers' reactions would be diverse. Captain Marvel, if she's part of the group, might be very matter - of - fact about it. Rhodey could be a bit bemused. Steve and Bucky, in the meantime, would be dealing with the embarrassment of being caught in what might be a very personal moment. The situation could also lead to some new conversations within the Avengers about personal boundaries and relationships among the team members.
Some fans would be extremely shocked and disappointed. Steve and Tony are two beloved characters, and their relationship is complex and interesting. Killing one of them off in such a way would be a huge departure from what many expect.
Natasha Romanoff would be disappointed. She values the unity among the Avengers and sees Tony and Steve as important pillars of that unity. She might try to mediate at first, but if the divorce is inevitable, she'd be sad to see the team's dynamic change so drastically.
Thor would be quite confused at first. Fanfiction is not something from Asgard. But once he gets the gist, he might say something like 'This is a strange Midgardian custom, but if it shows the great friendship of Steve and Bucky, then it is not all bad.' Meanwhile, Tony would be all over Steve, making snarky comments and asking if he's finally going to admit to some hidden feelings for Bucky according to the fanfiction.
Natasha would probably be very supportive. She's seen their relationship develop over time and would be happy for them. She might also be the one to help smooth out any bumps in the road during the wedding preparations.