I took his response to mean he is a virgin. When someone asks are you a virgin? Anyone who isn't a virgin is going to say no with conviction. He danced around the question like a virgin. He only mentions his senpai when Misaki mentions having a girlfriend, she wasn't his girlfriend either but he's mentioning her as a could've been girlfriend as he regrets not making a move when he had the chance. If he really wasn't a virgin he shouldn't of been flustered by the question at all. Instead, he's so annoyed by the idea that Yamazaki has a girlfriend that he goes to his school, a very crowded place in the hope of disproving it.
You do realize that 99% of NEET's in real life are virgins and that's probably the main reason most become NEET's right? So there's no such thing as a realistic show about a NEET that isn't a virgin, and the idea of Satou not being a virgin would ruin the entire point of the show (relatability). The reason he became a NEET was already not fleshed out enough in the first place but now if you assume he's not a virgin then it becomes even more silly that some random people laughing at him would ruin his life for years.