If you're an actor/singer/celebrity, coming out will affect your career now a hell of a lot less than 10,20,30 years ago. Especially now in the last 2-3 years when gay rights are such an extremely present issue.
That being said, actors are products. They are marketed by the people who produce products. They sell their image as much as their films (especially up and coming young actors whose purpose is to lure the teen demographic pretty much solely based on their looks). If a young hot male actor is being groomed as eye candy for girls to fantasize about, then yes it could potentially affect his career if he comes out saying "sorry girls, the thought of me passionately making out with any of you revolts me because I like taking cock up my ass".
If a smoking hot young actress came out as a lesbian I envision it wouldn't affect her as much since guys would just be fapping more to that fantasy.