Is when you consider yourself straight but extend your heterosexuality to transsexuals.

So a male would be attracted to a "male to female transsexual's" femininity or a female would be attracted to a "female to male transsexual's" masculinity.

However some feel it crosses the straight line to enjoy the transsexuals same sex parts too much.

Extended straight is about being attracted to a transsexuals gender expression. So it would generally be focused around doing it as, straight as possible.