A few things. One is to enlist the help of your boyfriend. These are his friends, too (I believe), and he should be presenting a somewhat united front with you. When the jerky behavior starts, he has to help you, and step in. "Oh, c'mon, iLevan is a great player!" That sort of thing.
I am not saying the burden is all his, but that's a part about being in a serious relationship, and you are if you're living together, pretty much by definition. So he needs to step up his boyfriend game.
You can also deflect at least some of that. It's a gaming situation, so it's competitive. "C'mon, you know my dwarf with his spells can beat your human archer anyday."
And be a better player. Smoke 'em on the playing field.