Is it normal to guess guys' penis size?

Right so no I'm not a paedophile and I've talked to some friends and they say they do it too-- does anyone else just guess how big guys' penises are..... Like I don't do it in a pervy way like "ooh looks like Allan has a big dick I wanna bang him" it's just like I can kinda tell the size of their thing and I try and stop myself doing it even though I don't say it out loud. I'm single too so it's not like I'm thinking about other guys rather than my partner. I don't know just tell me if its normal or not coz I think it's pretty weird but if other people do it then I'll feel a little better.