I wouldn't say ive had tons of chances to hook up, but ive definitely turned girls down. the first one was when i was in my senior year like you, this one girl liked me a lot but i just didn't feel the same way about her, and i had a huuuuge crush on a different girl, who barely knew i existed. i would smoke pot/cigarettes in her neighborhood, her dad would call the cops on me, rinse and repeat daily.
In college i spooned with a girl for the first time and was terrrified the whole time. this girl was an emotional wreck and i hung out with her because i felt bad for her, and after she confessed to being raped to me i felt obligated to be friends with her, and not that i don't feel bad for her but she made some BAD decisions in life.
another time in my sophomore year of college, i was selling pot to the UC Davis men's water polo team and my older sisters friend was at their party and she was rolling on ecstasy hard, and was all over me. I didn't do anythin with her because i would feel sleazy taking advantage of a drunk/fucked up girl while being stone cold sober. Though it would have been a fun one night stand, i would be prone to becoming too attached.
the last time i had a chance to get laid was when i was 22, my roomate/best friends younger sister came to town to check out colleges. she was 17. we hung out a lot, and were into each other, but once again i felt kinda sleazy being 22 and she was 17 (even though she was the one with ALL of the experience). Also her big brother/my best friend was RIGHT THERE. once again, part of me knows that would have been a fun one night stand, and who knows maybe i wouldn't be such an emotional wreck with low self esteem if i took my chance, but her brother is still my best friend and I'm glad i didn't ruin things with him for a one night stand. Ive been called dumb for not getting laid then, but i'm going to be a best man at his wedding. glad i didnt trash that friendship.