Curiosity---- I probably already know but what the heck...?

How the heck can a straight actor play someone who is gay? Heth Ledger, who I believe was not gay, played a rodeo fella with a family and a male lover in Broke Back Mountain (I didn't see it, by the way. Creeped me out alittle) I heard there were make-outs scenes. And that Robert Pattinson who played in the vampire-thing Twilight is in a movie, due out Nov. 2009, about a two poets who are exploring their writing and ual freedom. How can a non-gay actor handle the kissing, caressing, and what not whenever they are not that? I'm strictly curious because I would be uncomfortable even thinking about it.