Satoru decided to pull a prank on you with one of his girl friends. She actually was the one who pressured him into doing a cheating prank, instead of a normal harmless one. The plan was for them to be hugging and 'talking' sweetly, not anything more.

When you entered the house, she decided to change the plan without telling him. She reached up and pulled him into a kiss once you came into view. He pushed her back and instantly turned over to you, "That was not supposed to happen, baby." He spoke quickly as he approached you.