Thank you for the opportunity. Having recurrent and wanted intrusive thoughts is a hallmark of obsessive compulsive disorder. In what you describe, you are suffering from a version called "Pure-O" where the obsession is in the form of thoughts as opposed to compulsive behaviors like checking, counting, washing, etc. OCD is a very tricky and manipulative condition and one of the biggest things is that the more you fight against the unwanted thoughts, the more they become stuck and impossible to get rid of, It sounds like you are already learning this as the harder you try to let go of these ruminations by invalidating them, even ones you realize to be false (such as doubts about your sexuality), the more pervasive they become. Actually, concerns and intrusive doubts about ones own sexuality is common among sufferers of OCD and often termed Homosexual OCD or HOCD.
in order to get rid of these thoughts, you should consider medication as well as professional assistance. Ask your primary care doctor to start you on an anti-depressant such as Lexapro, Zoloft, Paxil or Lucox. All of these medications can help with the treatment of OCD to help the thoughts lessen.
The second thing you should consider is engaging with a therapist who specializes in OCD and offers a form of cognitive behavioral therapy called "Exposure and Response Prevention" (ERP). Under the guidance of a therapist, you will get up close and personal with these thoughts while developing ways to cope with them that do not involve avoidance, rituals, validations, etc. This type of therapy helps you safely confront the thoughts rather than run away from them, which is the common response for people when the ruminations are occurring.
In the meantime, the best way to help ease the OCD is to do exactly as you mentioned and to simply let the thoughts go. Directly concentrate on observing the thought as only a casual thought that your mind has, and then deliberately let the thought pass without giving it much care, pause or concern. Our very active brains are throwing at us lots and lots of *(often" useless information all the time but things that concern us tend to get stuck and are not easily released from our conscious. So again, when something pops ion your mind that causes you distress, literally acknowledge "oh hey there thought. I see you have popped back up. Come again some other time." And then get on with life. You describe this as ignoring and that is exactly what to do. These intrusive thoughts do not warrant your attention and should just be dismissed as meaningless, fleeting thoughts that are no longer allowed to affect the way you feel or behave.
I hope this is helpful. Please know that with the right treatment approach (ideally, a combo of an anti-depressant and therapy), you can free yourself from the taxing process of these unwanted thoughts. So have hope! And don't give in to the OCD by letting it control you. Thoughts are often very meaningless and you just need to get back to a place where you remember that these tricky and manipulative thoughts are meaningless as well.