Omg

Because he's desperately in love..

even I can't deny that I had once tried and pretended to be like another person for the person I like to like me back.. I even changed the way I look to suit that person's taste, change my wardrobe to his preferences.. we all do it for the sake of that STUPID LOVE...

We turned ourselves to someone else for the person to like us.. Seno here did just like that.. In his case, he did it literally..

while we only change ourselves outwardly.. I find it hypocritical when people say just be the way you are and let the people love who you really are.. And yet they dress themselves up prettily and wear perfume to impress others...

I do not excuse his lies but Seno did act selfishly for once after being treated by nothing but a sex partner by the person he loves dearly.. Which I think is commendable.. :)

Also, if you read it carefully.. Natsuki told Hakushima that he would believe lies like that if it is not Hakushima.. NAtsuki is also desperate to be loved by Hiro that he believe the body swap and disregard any logical reasoning.. He is in my point of view..also selfish..because he wasn't simply a victim.. HE CHOSE TO BE ONE..

I agree it was not very nice. It was very rude to him and it made me sad

No it was neither nice or right, and there is a definite trend in manga that all can be forgiven in the name of love. Think how any of us would feel if such a massive deception had perpetrated by someone we thought we knew who was, and had used what they knew about us to trick us for their own selfish ends. Because it is selfish to only care about your own feelings without caring how the other person will feel when they find out. And could you ever trust someone who would claim to love you but lie so outrageously to you at the same time?

yes that is correct！(๑╹ω╹๑ )