Maybe it's because he may have some feminine characteristics (being still early or ever pre puberty) but he's also getting old enough to start seeming less like a child. Just a theory.

First, I really admire you for raising your son as a single father. Not easy, but really great. And yes, it's sick to be having those urges towards your son. But human's sometimes wander into "sick" things, so the bigger issue is how to deal with it.

I think, (and I'm assuming a lot here, just going with my gut here) but I think it sounds like you're lonely, and have no outlet for your sexual urges, or your bisexual curiosity. So it's just backfiring and focusing on your son, since he's the only person you're that close to. Just like a guy can start fantasizing about his long-term female best friend, but more because he's started looking for (or longing for) a relationship in his life, and she's the closest he's currently got. So he's curious (subconsciously) to see if it can't become something more. And not because he's actually fallen in love with her, etc. It's not really about her at all.

I think you need to start dating. Focus all those urges on more healthy targets for it. And see if that doesn't make the weird urges towards your son go away. If it doesn't seem to help, or how you feel towards your son gets worse, then bring it up to your therapist anyways. Make it clear you just want help and would never ever do anything to your son. If he's a good therapist, then he should be able to offer some help. But it's better to be embarrassed and still get help, rather then let it get out of control.

But I'm just saying do that if dating and making friends doesn't help. That may be all you need, and if so, you can keep it locked away as just one of those weird things in life that's over. Get yourself a social life, some time for things to change and I think you'll be ok. If not, remember you have a therapist who's your best second option. And good luck, I'm rooting for you to pull through. =)