Don’t change who you are for them, that is denying yourself. Hopefully you will get a g/f and be seen with her, and that alone will correct their impression. As your circle of friends gowns and they know you as hetero, they will be the ones correcting those who make that assumption.
As pointed out above whatever they are picking up to make that assumption sometimes gives one greater access to the opposite sex.
If you have a close enough friend perhaps you can discuss that with them and gain insight as to why this is happening.
I don’t think I’ve been mistaken for homosexual*, but I have been mistaken for a woman (once for a specific woman). That was always cleared up as soon as they saw my face, because I’d make an extremely awkward looking woman.
Should you care that this is happening? I dunno. If it bothers you, “should” probably doesn’t enter into it. The only way to combat it that I know of is to show that you’re at least interested in the idea of sex with the women around you. You don’t have to be suggestive, but look at the right places often. I’m the nitwit who can’t help glancing at cleavage and butts when they’re presented, no matter their dimensions. Even though I’m probably more interested in a woman’s face as a selection criteria for a mate, the hormones are often in the driver’s seat. If you’re already doing this at the limit of social acceptability, I don’t know how you’re going to appear more heterosexual without becoming a dirty creeper or a slut. More “macho” (which I don’t really know how to define) would just make you seem more likely to be homosexual.
But really, if you’re not planning on dating a woman, what harm comes from them thinking you might be gay (especially if it shortly becomes evident you’re not)? In the end, it’s their screwed up gaydar.
*Unless being hit on counts, I’m not sure it does. Maybe they just thought I was open to new experiences? And slightly OT: I completely lack gaydar, the first indication that one might be gay usually comes to me when they suggest my participation, or just tell me that they’re gay. I get along just fine. If you’re not going to have sex with me, what does it matter who you’re lusting after?