They did it for the first time that night and I won't have it any other way. That said, I'm pretty sure Abe's got a misunderstanding here, Satsuki is not pregnant, especially not when Tamayo just warned them both knowing Naoto's own origin as a teen pregnancy baby. I'd expect them to use birth control methods after that talk with her. Also, signs and symptoms of pregnancy don't just appear the very next day. I'm expecting Tokui at least would be smart enough to know this if and when Abe contacts him.