Jon felt like he was in heaven, as Biff embraced and kissed him. He was so hot for Biff and Biff was so hot for him. It was paradise.
And even if Jon didn't look like a girl, he technically was one (considering he had female sex organs instead of male ones). From all appearances, he was a hairy, flat-chested, masculine-framed (but thin), short-haired girl. If no one knew better, they'd think that Biff was dating a guy. But Biff didn't care about that. He was attracted to people just like Jon. In fact, it turned him on that Jon looked more like a guy than a girl.
Jon dropped the wishing stone on his nightstand, then Biff pushed him onto the bed and told him to strip. Biff wanted to make love to his new girlfriend and he wasn't going to take "no" for an answer.
