If Ash marries Sabrina in a fanfiction, his Pokemon would react in different ways. Pikachu, being Ash's closest partner, would be the first to welcome Sabrina. It would probably jump on her shoulder and give her little electric shocks of excitement. His flying type Pokemon like Pidgeot might be happy as they see Sabrina as someone who can help Ash in a different way. Sabrina's Pokemon, especially her psychic ones, would initially be curious about Ash. But as they see how much Ash and Sabrina care for each other, they would be happy to be part of this new family of trainers and Pokemon.
Ash's Pokemon would likely have mixed reactions. Some like Bulbasaur, being the gentle and observant one, would be quick to accept Sabrina. It might even enjoy the new psychic powers that Sabrina could bring into their group. However, Squirtle might be a bit mischievous at first, trying to test Sabrina's patience. For Sabrina's Pokemon, Alakazam, being highly intelligent, would sense Ash's good intentions towards Sabrina and would support the relationship. Her other Pokemon would follow Alakazam's lead.
One possible storyline could be that Ash, after many adventures, returns to Saffron City where Sabrina is. Their relationship could start from Ash challenging her gym again but this time they see each other in a different light. Maybe Pikachu plays a role in bringing them closer, like it gets lost in Sabrina's psychic maze and Ash has to work with Sabrina to find it. This shared experience could be the start of their romantic journey.
Ash might be initially shocked. He has always seen Sabrina as an acquaintance or even a friend in some cases. So when she betrays him, he'd be taken aback. For instance, he could freeze for a moment, unable to believe what has just happened.
Pikachu might be very confused at first. When it sees 'Ash' (really May in Ash's body) acting differently, it could be hesitant to follow commands as usual. For May's Pokemon, they might be surprised to see 'May' (Ash in May's body) suddenly having a more battle - oriented attitude. For example, May's Beautifly might be confused when 'May' wants to train it for battles instead of focusing on its appearance for Contests.
His Pikachu would probably be very supportive. It would stay by his side as he studies, maybe even help him in small ways like fetching medical tools. It trusts Ash completely and would be happy to see him pursue something new.
Ash would probably be very shocked and hurt. Pokemon are like his family, and for Misty to steal one would go against everything he stands for.
Ash might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.
I think Ash would be angry as well as hurt. He's a passionate trainer and his friends mean a great deal to him. He might challenge May in a Pokémon battle, not to defeat her but to make her see how wrong her betrayal is. And during the battle, he would keep asking her to come back to their old friendship.
Ash would be extremely hurt. He's a very loyal person, and having someone he cares about like Lillie betray him would be a huge blow. He might initially go into denial, not believing that Lillie could do such a thing.
His Pokemon might be a bit confused at first. Pikachu, for example, is used to their adventures on the road. But they'll soon adapt. They could make new Pokemon friends at the school too. And they'll be there to support Ash during his school challenges, like in battles or tests.
His friends would be extremely shocked at first. Misty might scream and be really confused about what's happening to Ash. Brock, on the other hand, would probably try to stay calm and start thinking of ways to reverse the transformation right away. They would all be worried about how Ash is feeling in his new form and if he's in any pain. They would also be concerned about how they can still communicate with him effectively, since he's becoming a Pokemon.