Boyfried wants me to have sex with his brother?

He said his brother likes me and he wants me to have sex with him too. Why he wanna do that? He said they share things together and he had sex with his ex and his ex had sex with him too. I really love my bf and dun wanna upset him. Help!
Update: I did that with his brother twice cos i dun wanna upset him, and bf said since i done that with his brother now its ok to keep doing it. The thing is his brother told our friends we did that and now they called me whore. I regret 😢
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