Why do some people think I'm gay?

First of, I'm no homophobe, I'm not saying their is anything wrong with being gay or it's a bad thing but it irritates me when people falsely assume I am. A very good example is upon meeting my friends flatmates girlfriend for the second or so time she would giggle a bit when during conversations, and later once she left the flatmate in question explained she was convinced I'm gay so found it funny when I would talk about certain things. This is just one of numerous examples, and it's not like there insulting me or anything, just that they have a guenuine belief that I am gay. I have only got with a few girls before, and never have a serious girlfriend which can only reiterate their beliefs, but this is more due to myself being quite shy around girls than anything else, and as far as I know the exact opposite is the case with gay guys. I'm just a normal student who enjoys football, going to the pub etc, nor do I wear the sort of clothes that when you see you just know their gay. All my mates know what I'm actually like, it's with people I don't know well or know theough people who think tend to think this, but I just don't know why.