Yes, Marc is a negative stereotypical gay man, but UGLY BETTY is an equal opportunity stereotype presenter: there isn't one minority or majority group that it does focus on and satirize. If Marc were the only one getting roasted, I'd be protesting, but he's only one of many, and it's all in fun.
TOnight's show was hilarious and very wise, too. As someone who had an older sibling who was always the center of attention (sometimes not deserved), I know EXACTLY how Betty felt.