argue the poster is an inside joke of some sort, but why would Haim's character have that poster? In 1987 no major studio was going to imply that one of the teen-heartthrob stars of its latest summer blockbuster was playing a latent homosexual — God forbid — but Schumacher was gay and, like most talented people, had a sense of humor.
In my imagination Schumacher overheard Haim say something childishly homophobic on the set, then told the film's production designer, "Find a pinup of Rob Lowe and stick it on Corey's bedroom wall. Better yet, stick it on the door of his bedroom closet. Corey won't give it a second thought until after the movie comes out and he starts receiving some unexpected fan mail."