I had the same fantasy - totally straight, but just wanted to have a hard cock in my mouth. I finally got up the nerve and posted an ad through this fetish site, and got a hit. He came over to my place, a bit older than me (I'm 25, he was probably mid-late 30s). We agreed on mutual blowjobs and he went first. I was nervous as hell and mostly soft, but feeling his lips on my cock made me stiffen up real quick and I was soon leaning back and groaning before I unloaded in his mouth.
When it was my turn I just kinda jerked it a bit before thinking 'Fuck it,' and just went down on him. I was weird having a warm, hard, pulsing dick on my tongue. I sucked and jerked him the best I could, which was evidently pretty good. He even stopped me a few times to drag it out before he came all over my face.
Like I said, it was weird, but not in a bad way. I think it was mainly that he was a stranger and I wasn't completely comfortable with him, but if I ever find a guy friend who's ok with it, I'm willing to experiment a bit more.