Turning into Heat Blast is a great metaphor. As it stokes the flames of incest art between them.

One interpretation is there was a disconnect between the people doing the bumpers (they might have assumed Gwen was Ben's girlfriend because they only looked at basic character art and never actually watched the show, or were never told they have the same last name) and the people actually working the shows, and upper CN management couldn't be arsed to correct them.

IIRC this has been known to happen in some other material, like some early Simpson's merch and whatnot included stuff based on old concept material (like Bart's shirt being blue instead of red, and Marge having bunny ears under her hair which was a twist Matt Groening once wanted to use in the show really early on before scrapping it).