almost every guy thinks his thing is small, he could have a 12" one and he'll still feel like he needs to be better. It's in our society, the bigger you are the better you are, the more of man you are. They keep telling us if your not big then you can't get/keep a girl, she'll always look for a bigger guy. That's likely why he keeps asking, if you tell him he's big enough he'll feel more like a man and won't have to worry about losing you
unless it's literally dragging on the floor many guys are going think it's too small, or want a bigger one. He's asking this because he is incredibly insecure about it.
Sounds like your guy is self-conscious. Most guys are super self-conscious about it, unless they have an elephant trunk between their legs. Size doesn't matter as much as guys think though. There are plenty of positions out there for the well-endowed and the not so large guys. As long as a chick can actually feel it, it doesn't really matter.
For the same reason girls ask all the time if their chest is big enough: they're insecure. Simple as that. He needs reassurance that what he's equipped with is what you want. Also, p*rn usually has guys with abnormally large penises. He's probably comparing to that unrealistic size and finding himself inadequate.
He could just be looking for a thumbs up from you. Maybe he's insecure about it himself and wants you to tell him that he's perfectly sized down there. If he asks you this question during sex, maybe it's a turn on for him to hear you boost his ego.
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