Is it normal to be suddenly off-put by your boyfriend?

I've been with my boyfriend for over 3 years. He's the first real boyfriend I've ever had, and I love him very very much. He is kind and sweet (to me) and my best friend. He is crazy fun to be around, and very good in bed. However, I fear that I may not be so attracted to him as I once thought.

I came to this conclusion tonight, after he visited me after work for some late-night sugar. We were getting hot and heavy, I was pulling down his pants and getting ready to.. y'know. And there was this massive yellow pimple on his pubic region. Now, pimples aren't usually the kind of thing that gross me out. My boyfriend got terrible acne on his face when he was in high school (where we first met), and even now that his face has cleared up he still gets semi-severe acne on his back and butt. I don't care. It honestly has never bothered me in the slightest. However, just looking at that big yellow pimple freaked me out. I don't know whether it was because it was so close to his penis, or because it was so damn big, but I completely lost my lady-boner.
I told him (of course) that he had a mega-zit downstairs. He got rid of it (once again, popping pimples has never bothered me), and after he got over the minor embarrassment we got back to it. For some reason, I felt different. I was very aware of how heavily he sweats during sex, and how his O-face is actually kinda' weird, and how his hips and chest are so scrawny. I didn't want him to gaze longingly into my eyes, and I didn't want him to cuddle me after. I just wanted him to act like a man, finish screwing me, kiss me on the cheek and then leave. I was grossed out by his sweaty scrawny romantic self. And I think it's all because of that pimple.

Let me add that the "he's so scrawny" thing has crossed my mind before. My boyfriend is very tall and very very thin. I always liked tall lanky guys. They were, more or less, 'my type'. Despite this, I have sometimes wished to myself that my boyfriend would work out just a little. It's not because I don't find him attractive, but the fact that he's so skinny makes me feel less feminine. I have a 42 inch bust and a 28 inch waist, so my figure is very hourglass. I'm happy with my figure, and I'm happy with his, but every now and then I find myself wondering what it'd be like to have a pair of big muscular arms pick me up and throw me on the bed... but I'm getting into a whole other topic.

The point is that I'm suddenly slightly grossed out by my boyfriend. I'm worried that this feeling won't go away. Is this just a fleeting feeling? Will it subside next time I see him? Is it just a phase or is it the beginning of the end? I've never been in love with anyone before, I never wanted to, and I never thought I would. Is this how the 'end' start? Will my feelings gripe me even further until I am forced to break up with him? I don't know if I could handle that.
Please help me, I'm a novice.