Yeah if he slipped it in your ass he knew what he was doing. Secondly you can't just "slip it" in the ass without proper lube anyway. You should've felt him knocking at the back door and then you could've guided him to the front. If he slipped it in your ass, then he's not a virgin. You got scammed.
If you've masturbated before you actually had sex with him, you should tell him what you like and help him around. If he was also a virgin or pretty inexperienced he might not know his way around your lady bits