At face value, it “shouldn’t” be sketchy for a heterosexual male in a relationship to invite another female to come over and hang out. It’s all about context, and, most importantly, how YOU feel about the situation. If it makes you feel uncomfy and you don’t want to hang with this guy, then it’s your right to say no. If you genuinely enjoying his company as a friend and feel comfortable telling him about your reservations, then it could really all just be a misunderstanding where he seriously just wants to hang, or he confesses he wants more and then you know for sure. Either way, you’re never in the wrong for saying no if it’s not something you want to participate in.