Connor's a pretty aggressive character when it comes to love so he would go for the cliche romantic "kiss him in the moment" type moves to him. Rex is an eighteen-year-old virgin at that point so he wouldn't have any experience being confessed to, let alone how to reject someone. Connor is also Rex's friend and they know and trust each other, therefore Rex would be confused and surprised when Connor kisses him. He also doesn't want Connor to be upset, so he lets him off softly instead of outright saying no. And Rex doesn't even know if he'll end up with the love interest of his route at that point, so he doesn't want to reject his close friend only to find out that the guy he's got eyes for might not be interested in him. I'm not sure about putting in an option to outright say no because that wouldn't make sense for Rex's character.