The temple is making them feel their feelings 10 fold. Horus already is maxed out on how much he loves Seth, so he thinks it's normal to just feel his desire for Seth a little more.
For Seth, he's kinda foreign but he's like "fuck it. I feel something already that's already there (the temple isn't making me feel something that doesn't exist) ... I'm just gonna let myself be swept by how strong it's making me feel this."
He could say no because he easily came to his senses with the illusion. He willingly got her off of him and said, nah... I'm out. So it's not like he's fully unaware. I think he's very in control of the situation and aware of his feelings of jealousy and attraction.
He's just letting go of the reins right now. I was thinking the same before but as the raws kept going, it just made sense.
It kinda feels forced ngl