If somebody thought I was gay, I would be a little offended. Not because being gay is 'ewww', but because its kind of saying, "Hey, you dont seem very masculine to me."
As a straight guy, its normal to be proud of your masculinity, so if you think somebody is kind of insulting that part of you, its going to be taken as offensive.
Just like if you were tell a girl, "Hey, you look like a guy." Not that there's anything wrong with guys, its just that girls tend to pride themselves on their femininity, so saying they look like a guy is an insult to that part of them.