Alexis looks him in the eyes for the first time as he talks. She saw that this wasn't about pleasure for him. And understood when he said that it was for an heir. Someone that rich and powerful had to have an heir. But Seto Kaiba seemed like the type to not want anyone close. "W-will we be married?" It would make since, to hide something  as big as a slave. Even if he used petty words like concubine, she knew she was still a slave. She still had to do what he wants.