Sirius, having his own rebellious past, might first be a bit lenient. He'd listen to their side of the story. But if he thought they were being irresponsible, he'd become strict. He'd remind them of the potential dangers at Hogwarts and the importance of following the rules for their own safety.
Sirius might react with a mix of surprise and protectiveness. He would be like an older brother or father figure. He'd make sure they were safe and not doing anything too risky.
In a fanfiction, Sirius' reaction to catching Drarry could be complex. He has a deep bond with Harry, so his first instinct would be to protect Harry. If he caught them, he'd want to know what was going on. He might interrogate them separately. He could be especially tough on Draco, given his family's history. But if the story has developed Draco's character in a positive way, Sirius could eventually come around and see that their relationship might not be as bad as he initially thought. It all depends on the specific plot and character development in the fanfiction.
Ginny might be extremely embarrassed. She would probably blush deeply and try to hide behind Harry at first.
He'd probably be furious at first. He has a strong sense of loyalty and would feel deeply hurt by Ginny's betrayal.
They might start by saying that their relationship is based on deep love. Harry could mention that Ginny has been there for him through a lot of tough times and they just got carried away in a moment of passion. Ginny could add that they respect each other and didn't mean to disrespect Sirius or anyone else.
In many fanfictions, if Harry and Ginny were caught by Sirius, Sirius might be initially protective and concerned. He could question their actions, especially if they were in a situation that seemed dangerous or inappropriate for their age. For example, if they were out after curfew or in a restricted area. Sirius might give them a stern lecture about being responsible and the importance of following rules, while also understanding their adventurous spirits as he was quite the rebel himself when he was young.
In fanfiction, if Harry and Ginny were caught in bed by Sirius, there could be a lot of shock and embarrassment. Sirius might initially be angry or disappointed as he could see Harry as a younger person under his protection. He might give them a stern lecture about responsibility and propriety. Harry and Ginny would likely be extremely mortified. They would have to face Sirius' judgment and deal with the awkward situation. This could also potentially cause some rifts in their relationships with Sirius and among their friends if word got out.
In a fanfiction scenario, they could meet in Sirius's old family home at Grimmauld Place. Harry and Ginny might be exploring the place when Sirius appears. Sirius would greet them with his characteristic charm and warmth. He'd probably offer them a drink and then start a conversation about the current state of the wizarding world, asking Harry about his progress at Hogwarts and Ginny about her experiences in the Gryffindor house.
Harry might blush and stammer. He could say something like, 'Sirius, it's not like that.' and then quickly try to change the subject.
Sirius would probably be very worried and immediately try to help Harry. He might use some basic healing spells he knows.
Sirius would be deeply affected. He'd see Harry as a sort of surrogate family, and seeing Harry in such a state would make him feel like he failed in protecting him. He'd do everything he could to make Harry feel better, from getting him medical help to just sitting and talking with him for hours about what's wrong.