Ah, Man... it really is tough, when you are 'confused' and find yourself in your situation!
I've had a few experiences that really had me worried and got me harder than ever.
The first was when my best friend at the time, began to work out. So, one day, it came up in discussion. "Yeah, Dude, I have abs now. Wanna feel?" Now, this guy is straight, and just wants me to see his progress. I was excited of course, but had to downplay it. His abs were just on the verge of being visible. But, touching his stomach felt good.
Another time, also in high school, one of my friends and I had to work on a project together. We agreed to meet at his house in the afternoon. So, I go and meet him, ring the bell, and he answers the door. My friend is 6'2'', lean with muscles in all the right places... and here he is... shirtless. Crap... He was shirtless a good 30 minutes while I was there. You think I was paying attention to our project? Nope. I casually mentioned how he was in great shape, and without warning, he flexed his bicep, which was the size of an apple....
I guess my point, is, when 'falling' for straight guys, you just have to pretend nothing's going on. I mean, they are straight, so nothing will ever happen.