I'm baffled how any guy would not think it's a turn-on. Heck, I'm baffled how any guy would have to ask why it's a turn-on.
Basically a woman measuring my dick shows she likes it and appreciates it. On the surface it implies she likes what I do for her sexually, and on a subconscious level it implies she admires me, because like all guys I identify closely with my cock.