I let my housemates beat me in super smash bros. melee or project m sometimes just so they can feel better about themselves after they get beat.
That's kind of you. Most people would be upset about being beaten by someone they regard as worse.
I know, now they are insulting me all the time saying I am 'this bad' at the game since they've beaten me.
You are very nice for letting them win, but the way they are taking it is too far. Video games are just for fun, they have no reason to keep bullying you now that they won. Tell them to tone it down or you won't play with them anymore.
I tried that but they didn't listen.
They might just be taking their accomplishment too seriously, which isn't healthy. How about you show them your true skill, so they can be humbled a little bit? You don't deserve this bullying when you were being nice to them.