basically, take it the same way you'd take interest from women (apart from the returning the interest part

) if the guy is ugly as hell, then he might just be desperate and trying anyone he sees. if the guy is good looking (in a purely aesthetic way, not a "you find him hot way" but you know, if he looks like the sorta dude who'd get a lot of girls if he was straight) then take it as a compliment. best reaction is just to say something like "i'm flattered, but i'm straight." if they carry on harrassing you after that just enforce the point strongly and tell them to get lost.