Hi gotmilk,
Welcome to the forums.

I don't think that you're sick. I think that what you have is a remnant of childhood curiosity combined with adult admiration, kind of like "how does that thing
really work?" You mentioned that you have obsessive thoughts--so it's likely that the more you try to push it away, the more it pushes back and wants to stay in your head.
I am a bisexual female, and I think that the penis is a work of Nature's art. I also think that if you turned the attention more to your boyfriend's genitals, it would reinforce the grown-up version of the curiosity and admiration. I think that curiosity and exploration can be a healthy thing in a relationship, and your boyfriend might appreciate the extra attention.

I think that if the thought persists, or if you have other intrusive thoughts, it probably wouldn't hurt to seek help for them. Most therapists are smart enough to distinguish an obsessive thought from a sexual attraction disorder.
Just to ease your mind, I'm going to share an embarrassing story with you. It plagued me for
years, and I discussed it with at least 2 therapists and several trusted people. Needless to say, I empathize with you when you say you have obsessive thoughts. (MATURE CONTENT FOLLOWS.)
One night, about 10 PM (dark), I saw what I
thought was a bridal party leaving a place that served alcohol. So I made some off-hand (and lewd) comment about one of the "women". As my friend and I drove closer, I realized that the girls were like 8 years old!!!

Bad lighting turned into approximately $500 in therapy expenses!
But at least I KNOW I'm not a pedophile. For example, I actually get turned off by full bikini waxes, both on men and women. Something in my "cavegirl brain" says, "Nuh-uh. That's not an adult." To me, things like that are a sign that I'm wired correctly in that department. I'm sure that you, too, have you own, rational signs that your sexual behavior and thought patterns are normal.
I apologize for the rather graphic explanation, but I hope that it will set your mind at ease. As far as I can tell, you're fine.

--Frayed
Do not take my advice before talking to your doctor/counselor/other professional. Depending on where you live, you may be able to find free, confidential care. Most importantly, sometimes your shrink can be wrong. Get a second opinion.